So i have a 502 efi (1993) merc motor and it loses power once ive been running for about 15-20min at WOT. It drops about 500-1000 rpms. Checked fuel pressure, reads right around 30psi. I had the fuel pressure gauge on the schrader valve all day while i was running and it never dropped. however the engine would just slow down about 6mph out of nowhere, then it would be fine again. This happens to me the last few times ive been out. Please give me some guidance. I don't have access to a scanner so anything else I could check would be great.

TPS? IAC? air flow sensor?

Please help as this is so annoying!

Try disconnecting the knock sensor and trying a run. The knock sensor shorts to ground and pulls timing out of the motor when it detects detonation.

You may be detonating after 15-20 min WOT.

Do your plugs look OK?

Plugged injectors would let the fuel pressure on the rail show OK but not enough fuel going to the intake.

Pulled plugs, all are burned evenly. A little rich but they run rich from the factory. If i unplug the knock sensor and it runs great what does that mean? Could i cause damage by running with it unplugged for testing purposes? What would be a symptom of detonation? Thanks for the help

Recheck your total advance timing, may have to much total advance at higher rpm