I changed the heater core and it sat for a mont. put everything back together correctly and now the car turns over, stays around 2000 rpms for a few seconds then dies. It is a 1995 with a 3.4 v6 manual.

I am having the same issue with my 94 z28. it was sitting for a long time while i was in Iraq but driven occasionally by my father. it will sit at around 2k rpm then skip down to about 1k rpm then die, and it start very rough. any help or thoughts? there is also a slight knocking from the engine I'm assuming pistons. When i am able to drive it it is super sluggish all the way up to 3k rpm then seems to run fine but still rough. i changed spark plugs and check all cable connections but it still is having issues and i don't want to take it to a shop if its something i can fix myself. please help

My first guess would something with a hose that has vacuum on it. GM loved using hard plastic lines that get brittle few quickly. Might not be crack but loose at an end. My old V6 Grand Prix was famous for this. I just always thought it ran like crap. Then I started finding lines that had come loose and/or broke. I was amazed how much better it ran.Felt bad I'd let it go that long.