Engine RPM falls off. Long.

[LarryLI]

The boat is an '85 Carver with twin 5.7 Merc inboards. I have been having a problem with the starboard engine. It starts and runs fine, idles well, oil pressure and temp are normal. When put under load the engine will start to lose RPM's and stall. Re-start is no problem. Turns out even without load if I run the engine up to 2000 RPM it will start to lose RPM and stall. Port engine runs fine. I thought perhaps the fuel itself might be the problem, as I haven't been able to use the boat much due to illness, so I tried running the starboard engine from the port tank and had the same result. Next step was to change the water separating fuel filters. I changed the filter on the starboard engine with no problem, caught the small amout of gas that came out when the filter was removed, no problem. Removed the filter from the port engine and was surprised by the amount of gas that came out, really startled me. Caught it all, no environmental issues thankfully. Two questions. Why the difference in the amount of fuel dumped while changing the filter on the good engine and of course the primary problem remains. The starboard engine will not run above idle. My thought is the fuel pump. Sorry this ran long, any suggestions would be appreciated.