I just got a 2005 Grand Prix used (base model, 3.8l non super), it has a flashing CEL but only if I drive over 30 MPH. If I stay under 30, it stays on steady.

Had a mechanic check it, it showed P0301, P0302, P0304, and P0305. Cylinder 1 and 4 had heavy misfiring, and cylinder 2 and 5 had some misfiring. 3 and 6 showed no misfiring.

The sound of engine seemed very nice, not like it'* misfiring. The coils (all 3) were shifted around, the problem did not follow so that ruled out faulty coil. I had the ignition module, spark plugs, cables, and MAF sensor replaced. The CEL is still on and still flashes at over 30MPH.

Fuel was checked, the flow was good and PSI at idle, 2500 rpm, and while driving were all within spec. Cat converter is almost new and vacuum check didn't find any problem. Compression testing was also performed and all 6 was within spec.

One thing that was noticed, if the MAF was left unplugged, the CEL does not flash at higher speed. I get the impression something is interfering with the air flow or fuel but where? Or is there another sensor somewhere I am missing?

Any idea? Car repair is not really my thing, I kind of lost it way back when computer were added to cars. I remember back when you could tell right down to which bolt was loose by listening to the sound and tasting the dip stick. Now I need a mechanic that has a $4,000 OBD reader and it still can't tell us why the misfiring seems to occur, particularly when it doesn't sound like misfiring.

Check the ignition control module:

Part 4 -GM 3.8L Ignition Module Test and Crank Sensor Test (3X, 18X)

if not the icm or crank sensor, when the lower and or upper intake manifold gaskets go they can drip oil or coolant into the cyl and cause misfire. when mine went the second time you couldnt tell it had a problem if you didnt see the light and i had a little coolant loss that i couldnt find

The gasket is aluminum and shouldn't be having problem at all. The cam sensor was part of it but all I needed was 15 minutes to make the car do a relearn on cam sensor. No new part needed after all.

Some of the new parts would probably be needed eventually since the car has 133k miles and I don't have any repair history of before I got the car.