Almost a year ago I had the transmission completely go out on me in the middle of an intersection. We had it rebuilt and everything was great until a few months ago. It started to do a "BANG" start, but only once or twice a month. Then it stopped until yesterday when my wife took it to work and it wouldn't accelerate and then slammed into gear, really hard. Today it would accelerate at all. I really need some feedback here on what the problem could be, anything helps.

Please always with your post list the vehicle info like 04 LeSabre etc.
Hopefully since you had it rebuilt, it came with a warranty.
Did u take it back to the re builder yet?
Please inspect the transmission fluid level, when hot in park on level surface.
Is the fluid clean and on the full mark not low etc?

assuming the fluid is correct, it sounds like the torque converter causing your accel problems. as for the shifting issues, is sounds like a stuck check ball in the trans.

I checked the fluid and it look fine, right level and color. I had it towed to the shop were i had it rebuilt and he test drove it and hr said it was fine and drove perfectly, he drove it later and it did shift hard, but its now moving. Any ideas?
2004 Buick lesabre limited

Also the mechanic used a break in box to bypass the computer to just check the trans and it ran just fine

2004 Buick Lesabre limited

Sounds like he doesn't want to stick to his warranty. Certainly you aren't going to just taken it to him and make up symptoms. If he doesn't honor the warranty, and you have someone else check it out, make sure you stick him with the cost of the repairs. It does sound like you've got a torque converter lockup issue on the acceleration. What is your vehicle/transmission?

Nvm re-read your last post.

I have hard shift and installed a Transgo shift kit into the 4T65-E transmission'* accumulator. BUT...before I installed the shift kit this is what I did to determine if it was the torque converter or the pressure solenoid inside the valve body which are known to be problematic. You will probably want to look inside the transmission pan to see if there are any particles inside there but before u do read on...
First off I had the transmission and all codes scanned are car. Yes in memory it showed the dreaded P1811 long adapt shift code indicating it was solenoid problem.
Since accumulations of varnish can cause the solenoid to stick and check balls inside the valve body to stick, I added one full can (recommend only half a can) of Seafoam Transtune which works awesome to remove the varnish. I drove with this product in, including a few hours of highway driving to get it hot to allow the job of cleaning for 800 miles. Trans Tune | Sea Foam Sales Company No further major hard shifts like before were present after using the TransTune product. Note with these transmissions it is a good idea to use the transtune anytime before a pan drop for 800 miles, then install new Dexron VI (6) fluid and filter.
Ok so onto the shift kit which is accessible after the transmission pan is dropped.
The Transgo shift kit TransGo | 4T65E: SK 4T65Ecomes with instructions and makes sense once you have the accumulator off easy job. I did it in less than 30 minutes taking my time.
Note I also use LubGard Red which keeps the fluid much cooler, in with the new Dexron 6 fluid.
LUBEGARD Automatic Transmission Fluid Protectant

Here is the link showing photos of the accumulator TransGo shift Kit install.
I should have enclosed this link above oops..
https://www.gmforum.com/buick-172/4t...-302566/page5/

As an update I had to replace some wiring on the for the computer to read the codes that it was giving and it was codes p0785 and code p1860 which are the codes for the shift solenoid and the torque converter. Thanks to everyone that posted your help was much appreciated thank you
2004 Buick Lesabre limited.