95 LeSabre 3800 problem.... help?

I have a 95 le Sabre with the 3800 tank of an engine in it.. I absolutely LOVE the car... bought it used with high mileage 130k for 2 grand... not really a bad deal considering the condition of the car was excellent inside and out. Everything works perfectly, except for ONE annoying problem that keeps comming up... now that the weather has gotten warmer.. the car starts and runs beautifully till I get to about 45mph, then if I hit the accellorator to pass someone or just gather more speed in general, it starts sputtering like it cant keep up with the fuel supply and barely keeps going at speed. (making sense so far?) Now.. Ive taken it to a dealership in the area to have em fix the problem.. first time they replaced the sparkplug wires... and a couple vaccum lines that were bad. road tested it. and had no trouble.. till I road tested it.. same problem. Now.. I KNOW its not the fuel pump (the fuel has plenty of pressure.. and therefore the filters arent clogged either) soo I took it back.. this time they replaced the 3 coils.. and the sending module. STILL no good... Ive spent 1000 bucks in the last week trying to get this problem solved.. anyone have any ideas where to look?

So, it doesn't do it at all when it is cold out or when the vehicle is not warmed up yet? If you jump on it then, it works fine? Is it definitely heat or temperature related?

No I never noticed any problem when it was 20 below zero... lol... welcome to wisconsin... weve had a warm spell lately, and Ive really only noticed it then. but now that it occurs.. it seems to happen no matter what the air temp is.. 55degrees.. or 95 degrees...

I had a similar "problem" with my 95. It would bog down a bit before downshifting and accelerating after I would hit the pedal at higher speeds. I took it in and had it test driven. I was told it was the nature of the transmission. The lesabre has a lock up transmission which "locks" into a gear after reaching a certain speed for a sustained amount of time. Slight acceleration does not cause the tranny to immediately down shift or the engine to immediately rev higher. When really giving it some gas it takes a second or two for the transmission to recognize that in order to reach the desired speed the engine must down shift and increase revs; so it bogs a little bit before it accelerates. There really wasn't anything wrong with my lesabre, it was just the way the transmission operates. I get around the problem by lifting my foot off the accelerator completely and letting the engine almost reach idle for a brief second while I coast for a bit. Then I give it the gas. This resets the transmission and the engine doesn't bog down.

It sounds like your problem may be a bit more severe than what was happening to me, but because of the similarities I'd take a look at the transmission.But try the foot off the pedal technique first.