It just doesn't run right-200 Merc

Well, If anybody can help I'd appreciate it. I have two problems, don't know if they're related or not. Prob 1. Runs real rough starting around 1000, clears out about 2000. It's as if one cyl is not firing or like one cyl is so far advanced it's trying to run backwards. (makes for very uncomfortable trolling!
Prob 2. It will run 4600 all day and it will run 5600 (45-48mph) for 30 sec to 2 minutes. Then it quits like it is fuel starved. If I pull back the throttle to neutral it usually keeps running and after 30 sec to a minute I can run again (up to about 4600).
Now, I just got it back from a mechanic who rebuilt the carbs and fixed a third problem (flooding and skipping on one or more cyl at high speed). These other two still exist. Wk before last in the gulf I shut it off when it "quit" at high speed, pulled the plugs and saw 1 bank was clean and the other was dirty. Hot Dog! it must be the switchbox! Wrong. I tried a new one on port and then rotated to starboard with no difference. I run off three different tanks w/two hoses/bulbs--no difference. Comp is 120 on all. All check valves to the bleed hoses work. All main jets are OK. I'm spending more money on plugs than gas. I've changed the impeller and running about 20psi at 4500 to 5000. Have a stream out the tell tale. The stator was replaced last year. I do get a "healthy" spark on all plugs/cylinders at cranking speed. Etc., Etc.
Any ideas? ???
Trollercoaster