rebuilt engine misses after 375 miles?

[rfb427]

Hey hows it going, first thing is first this engine is a 5.7L 346 Out of a 1999 firebird formula was removed and rebuilt completely and a more intense cam installed (comp cam), it cranked right up on initial start, oil press high at first but it dropped well below 40 after warming up. A service engine light appeared, this code came up right from start up. P0300 random multiple missfire, but after a couple of starts it would go away, and this happened like six times since the engine has been operational, off and on this light for service engine it won't go off so i keep driving it and all the sudden the engine starts to spudder and almost die thern the service engine light began to flash. Now it misses hard on cylinder 7 and a couple other cyl's are slightly missing. Fuel pressure good, fuel good, plugs good, tps good, maf good, mechanical function looks good, leak down test good, compression good on missfire cyl. I am having a heck of a time with this one any tips or help would be appreciated, thanks.

[wrightz28]

What year car did you put the motor in? Did you use the engine management system from the donor car?

[rfb427]

I put it in the same car it came out of a 1999 firebird formula. Thanks for your response, however instead of the stock valve spaings i used a dual valve spring and found Cyl 6 to have a broken spring on the exhaust so i replaced it and all is good for now.