transmission question

hi everybody, i have a 1995 chevy lumina apv. my problem started a couple of months ago, and it finally has drove me crazy trying to figure it out. about 1/2 the time the transmission works perfect and the other 1/2 it wont shift into 2nd gear unless you let off the gas pedal. then it shifts into 2nd gear fine. but if you give it much gas after that it tries to shift back down into 1st again. the van has the 3800 engine in it. does this sound like the modulator valve or could it be something else? thanks in advance

If you have good vacuum to the modulator, scan it an check the tps functions.