1989 GMC C1500 Stalling, Hesitation and Rough idle

I have an 89 GMC C1500 with a 305.When I bought it the guy said it had been sitting for 2 years. So I had to take the heads and intake manifold off for cleaning. I have it back up and running, but it runs rough, stalls often, and hesitates under acceleration. I ran a compression test and it reads 200 and above for all cylinders, I adjusted the timing, ran an OBD 1 on it and it says everything is fine, there is just one thing though, the check engine light keeps going on and off. I replaced the air filter, fuel filter, fuel pump and sending unit, spark plugs and I drained the old fuel and added fuel treatment, but it hasnt improved in any way. So any comments would be greatly appreciated.