Engine stumbles at speed

I have a GMC Safari with a 350 chev engine from a 92 pickup. Recently it has started to stumble and buck at a constant throttle setting. At idle, it seems fine, but at idle in gear it will stall as the idle keeps getting lower and lower. It seems fine under acceleration but as soon as I start to cruise, it jumps and stumbles, what is causing this?

So far I heard it could be the TPS, the ERG, the MAP sensor...anyone have something a bit more definitive?

cheers!
Diva