Z34 Lumina

1992 Chevy Lumina z34 5speed with 230thousand miles v8. When you start the car it dies. It does not stay running by itself. When you start out its really jerky without moving your foot on the gas or clutch. Sometimes it wont even reach 3 grand it wont even move. But once it does hit 3 grand it takes off and runs perfectly fine. We took it to a mechanic and he ran propane through it and said it ran fine. So he thought maybe a dirty injector or something and suggested to run a couple tanks of gas and some lucas thru it. Well 4 tanks of gas later nothing has really happened. Sometimes it will run absolutly fine when you start it up but way more times than not it doesnt. I dont think it would be something thats clogged like he thought because otherwise it would be firing randomely not just take off at 3 grand. We are completly stumped and dont want to just start chaning parts that are $100 a piece. Any suggestions?