Port engine loses power

I have a 2000 340DA with 7.4l mercruisers, Bravo 3s.

Late last season, I started having issues with my Port engine. I would be cruising at my usual 3200 rpm for a while and then the port engine would suddenly lose power. The engine would not stall but it would not maintain the 3200+ rpm cruising level. It would drop from 3200 down to the 1000 - 1500 rpm level. If I turned the engine off and restart it, it would perform fine for a while and then start this "lose power" scenario again after a few minutes.

FYI: At the lower (1000- 1500) rpm level, the engine runs fine, no issues at this lower level.

The Yard looked at the issue and they tried to clean out the fuel line by disconnecting it and blowing it thru it (they blew thru it with their mouth, they did not use pressure from a machine/compressor). I also added Sea Foam to the Port tank at the end of last season. These 2 solutions seemed to work, for a while, at the end of the season. But now, the same problem has returned this season!!!!

The yard believes it is a "fuel issue" and is going to try to blow out the system using a compressor that will provide more force to clean out the fuel lines

Has anyone:
1. Experienced this type of loss of power issue? Any thoughts/solutions/suggestions????
2. Any thoughts about using Sea Foam as a fuel additive? I've read conflicting opinions/thoughts about the use of this product. Some say it is great! Some say it's harmful to the system!

Any help is appreciated ....

Start with the vent line.
If it’s clogged, the engine will starve for fuel after running for a bit.

next time it happens, immediately remove the fuel filler cap for that tank and see if helps.

Much older boat but very similar problem when we first got her.
After running fine for a bit, the engine (starboard in my case) would slowly start coming down in revs until it backfired!
The rpm would then jump right back up and the engine would behave normally, at least for a while. Over a 50 mile trip home, this happened 6 or seven times in the course of about 3 hours.

I did the sea-foam before we left along with changing out the three filters / water separators for each engine.
In the end, none of that worked and I had to rebuild the carb to cure the problem.