My 87 5.0 idles and drives good but if you try to put your foot into it it falls on its face around 3k and starts to back fire and lose speed I've replaced the maf ajusted the tps my fuel pressure up around 42 psi I'm at a loss and don't know where else to look. All opinions are welcome.
It'll run alright at idle with the timing being off. What it possibly sounds like is that the timing is too retarded, so when it reaches the ECU's set point for full advance, it is still too retarded to fire at the optimal time. That's assuming backfire through the intake. Backfire through the exhaust, the opposite could be true and the plug is firing the unspent gasses as they leave the exhaust valve. I'd start with a timing check and verify what it's up to. If everything is fine, I'm sure there will be codes involved so check for codes after verifying timing if that's the case.
I agree with checking codes too. Does the car only stumble at 3k and then pick up again and keep going or does it just fall on it's face at 3k and you can't really accelerate quickly beyond that? If it won't go past 3k I'm going to say it's a fuel pump issue or gunked up fuel filter. It will only feed your engine at low rpm but when the demand is too much it can't keep up anymore.
Does the problem occur al the time or just on a warm engine?
If only on a warm engine, then you have TFI module or PIP sensor problems.
If any time, then you have other problems. Codes may present even if the CEL (Check Engine Light) isn't on so dump the codes.
Dump codes sticky.
Look at the top of the 5.0 Tech forum where the sticky threads are posted. One of them is how to dump the computer codes. I highly suggest that you read it and follow the instructions to dump the codes.