Motor Drops RPM's

[Cjc5602]

Hi there,
I've got a 5.0 Mercruiser with the thunderbolt ignition and a 2 barrel mercarb. Heres what's going on. The motor starts just fine and idles smooth and revs up all day.......no stumbling nothing. I'll take her out and run her up to 4000 rpm and after about 15 min the RPMs start to drop. They'll drop down gradually and then go back up and then inevitably ends up cutting out completely. She's looping. Now this Saturday I ran her for 2 hours at 3000 rpm going up River and she never did it, but Sunday coming back she acted up again.......ran for 15 min, cut out......let her sit for 10 min then she'd go for another 15 min.
Here's what I've done to try and solve the issue.
Brand new Carberator, new plugs, wires, distributor cap, rotor, ignition positioning modual, and a new coil. I installed a new fuel pump too and all new fuel filters. I've pulled the fuel line and it's clear and has plenty of flow and the anti siphon valve is new. I've checked the compression on the cylinders and they are all right at 150psi.
Any suggestions

 

[Scott Danforth]

Open your fuel cap. If the issue stops, your vent line is plugged

 

[Cjc5602]

I forgot to include that I did check the vent plug. I opened the cap and blew air through it and could hear it gurgle. Plus on Friday when I was filling her up I could see fumes coming out of the vent plug due to how the sun was hitting

 

[Rick Stephens]

Your list is pretty complete of what you have replaced and tested. About the only thing I can think of to try is install a fuel pressure gauge inline and see how pressure to the carb is holding up during sustained use.