2000 LeSabre sputters when warmed up

my 2000 LeSabre started to lose acceleration power while driving, noticed that it seems to be backfiring or sputtering like it is a fuel problem. from a take off I can accelerate until about 25 mph then its like it doesnt want to change gears, I let off the gas and reapply gas it will resume accellerating, but only to about 50 mph, then I can do the same. but top speed seems to be about 55 or 60 mph. is this a sensor or an adjustment? Is this somthing I can do at home? any ideas would be great. thanks!!

I had a MAP sensor on a 98 Aurora give me these exact same symptoms but i'd suggest you have somebody put it on a computer to see what's up.