No oil pressure
 

I have a 97 SS camaro. Well, i got the car all back together and running after replacing the rear main seal, oil pan gasked, and timingcover gasket. I took it out lastnight and it screwed up. The check guages light came on and the car shut off. I looked and i showed no oil pressure. So i switched out the oil sending unit today and it showed pressure for a half mile. Then i tried to step on it and it died. I tried to start it and it took about 10 tries to start. It wouldnt go above 40 mph and i was steppin on it and it wouldn't go anywhere. I could rev the engine and it would show a little pressure. The only codes that came up where multiple/random cylinder missfire. Any ideas?