Guy's I need advice?

My fairlane has a rebuilt v8 302 engine. I have a problem where it will run fine for 20 to 40 mins, then start running really rich. With the battery freshly connected on start crisp throttle response, runs how it should.

Then, suddenly it will run reaal rich. If you disconnect battery to reset ecu and restart engine it runs fine then after a while rich all of a sudden.

I am using a J3 chip mapped for the engine. All sensor voltages check ok. We have tried a second ecu which made no difference. (Actually it ran a bit longer before it ran rich with the 2nd ecu)

Has anyone come across this problem before?

Cheers

MAF sensor failure will make it go really rich. Or o2 sensors going bad maybe?

How do you know it's going rich, what are the signs you are seeing?

It was a faulty fuel pressure regulator.
I was nearly at my wits end.
The pressure was waaay to high. Anyway, for now we have used the old one from the other 5.0 motor. Even though 220kw is meant to have a revised fuel pressure it runs great. The independent afr meter reads perfect mixture.

Thanks to Jason from T.I. Performance for your advice. I'll be buying the rest of the kit now, so i can start learning to tune.
Guess ill be studying retarding ignition curves, coz i just bought a nitrous kit from the states. How long will it take me to split the block, blow the trans or pop the diff and in what order? i left out melting the internals coz my tune will be perfect lol