i have a 1986 305 tpi and i will start it and will idle fine at 1200 rpm but when it settles down to 800 or so it starts to sputter and every now and then will die but as soon as i give it gas its fine driving down the road, also the cat is gone and the smog pump is bypassed, i dont know if that influnces it or not but i figured id throw that in there.

throttle position sensor possibly?

Sounds like the TPS or IAC. How is it when you drive it? Stumbles?

i would definitely replace the TPS or the MAFS I had the same problem and I replaced those 2 sensor and now she runs like a champ!!!!!!