My GTP seems to miss under hard acceleration.
Not sure if it is a ignition of fuel delivery problem
the car seems to idle fine and seems to accelerate well if I give it the pedal slowly and lightly but if I want to get right on it and give it the pedal too hard and fast she starts stumbling.
ICM problem?
Fuel delivery maybe?
I have changed out the coil packs, plugs also
changing the wires today but kinda concerned of a bad ICM
would a bad ICM cause this, remember she starts up and seems to idle fine
Any help would be greatly appreciated

This problem has been coming up a lot on here this winter. It could be a lot of things. A bad ICM would cause it to do that, but another member had a very similar thing happen and it ended up being bad springs. I would say, see if you can isolate a cylinder and then go from there. If it is still a random misfire, then go for the ICM and on. It also doesn't hurt to run either Seafoam or 44k through your gas system to make sure that it'* not just the fuel system. Just some tid bits that I've picked up when I dealt with the same thing a couple weeks ago.

Have the car scanned for codes. A p0300 wont help much, but a P030x will tell you if it'* one cylinder. You can also test your fuel pressure. Should be around 48 psi. A deing fuel pump could loose pressure and cause a lean condition which can wreck your engine. Also, are you running 93 for fuel? 87 will kill your engine. A clogged cat can cause a miss under heavy load, as can a bad valve spring. But go threw the ignition system before you start moving on to mechanical problems.

I know what you are saying about the fuel grade, was running 87 more often then not and i am sure it had something to do with the original motor failure.
The stumbling, I have going under hard acceleration seems to be ignition related. Today, I changed the plug wires and did away with the heat shields around them. Also I have been running 93 in it with a bottle of Lucas fuel system cleaner,can't hurt right? Now it is running much better. It will still stumbles or hesitates a little now and then but nothing like what it was doing before the wire change. The car gets up much better now so I think I am the right track. I am thinking, maybe, pull them $50.00 plugs out and trying something else. Any Ideas what I should try?
I will check the fuel pressure and the misfire counts when I get a chance to get to the GUY with the right tools for that check. What about this ICM? Would this have any bearing on a random hesitation under a load like this?
I going to keep pocking around and changing out things, seeing how I have my old motor (that didn't have this weird problem) and everything on it is good except for the ugly #4 cylinder.
Keep the ideas coming and I will let you know what I come up with.
Thanks All and Happy New Year!!!

Just an other note to this aggravating problem. I have the basic scanner and I have been checking for codes and have NONE! No engine light on or even blinking when it stumbles like that. Or at all so far.
A code would help. I have pushed it trough the stumble to try to get a code but nothing so far.
Crazy right?
This thing is driving me nuts. Good thing I love this car or I would get rid of it and stop spending stupid money.
thanks again

Did you do a CASE learn? That tells the PCM where TDC is to get the timing right. If you have the other engine, pull the ICM off and swap it. No harm in trying.

I have a 1996 Buick PA Ultra that required a new fuel pump last month. Last week it was missing too (at first no check engine light, but it got bad enough to set a code) - took out my Genisys tool and found that # 2, 4 and 6 were missing. Also it set a P0300 code so that lead me to look for a misfire. all three plug wires going across the top of the engine. Thought it'* time to replace them - did so with a set of NAPA / Belden wires and it fixed the problem. I'm running the NGK Iriduim plugs too. I would suggest you test the reserve fuel pressure to see if the pump can deliver the amount of fuel required. I believe there'* a YouTube video that shows the test procedure... Good Luck

Thanks for all the great info. And ideas guys !! So I should have posted the results of what I have done that seems to have fixed the problem.
Sorry about that !!
Anyway ...after changing out the plug wires and leaving out the silly heat shields around the plug boots as I posted before and running some good fuel system cleaner with GOOD gas the car is running normal now !! Finally!!! Its was making me crazy!! Can't believe I payed the auto shop $3000 to replace the motor and I still had to fix its myself ....maybe. I shouldn't say this ... But don't bring yours ride to Image auto sales and service ....in fitchburg ma. I don't think they are any good !!!
So now that the car is running better (not really the same as with the original motor but running good ) I am now looking at an other stupid problem...evap leak!!! Replaced the gas cap...removed purge value and cleaned it and put it back in ...at first it seemed to be fixed but just got the light back again .!!!! I am going to replace the value today and see what happens... Any ideas where else to look?
Again thanks for all the help guys ...this is a great site for these things
Scott *.

Start looking over the vacuum lines. A small leak will give you that code.