i had a similar problem and it was cuz my alternator belt was loose and causing voltage issues so my fuel pump wasnt getting what it needed all the time. i would set the psi to like 44 and drive for 5 min then it would go down to like 20 then i would reset it and it just kept doing it over and over it was fucking annoying

 

i replaced my whole entire fuel system including the tank in the process it was not fun

 

What do you think about the last line of post 7......fuel pressure regulator?
Have you tried swapping regulators with someone to see if the regulator is bad?

If you swap regulators and the pressure is still inconsistent, it may be the fuel pump.
A new fuel pump is no guarantee that it is a a good one and with a known good regulator
and.......... if all your filters are good, the pressure should get back up.

Next, check the gas. sometimes I have seen the gas/ethanol "separate" and it looks like
rusty water when the car sits for a while.-Bob

 

He already did all that. You could be getting voltage drop at the pump

 

B18,
Tell us a good way to check it and a way to fix it. Thanks.-Bob

 

put a volt meter on your battery,check your wiring,etc.are you 100% positive you did everything right when you put the motor in?

 

Change these with known good ones:

regulator
main relay
wiring
pump....in that order do to ease and price

and if your "old" regulator was making a noise....that doesn't count as a "known good" regulator.

 

You check the fuel feed line?

 

Is the engine mechanical timing correct? Is the ignition timing correct? If you're shooting flames out of the intake, the engine runs when it's cold, and it looses power when it is warm, you could have a major timing issue.

 

The other thing that makes me think that it is a timing issue is the drop in fuel pressure. If it is out of timing, mechanically, it will not create the correct amount of vacuum required to keep the fuel pressure up when warm, after the computer advances the timing. You said you just put the engine in...you could have put the belt on incorrectly. Always set the timing to the oil pump mark, not the mark on the plastic cover.