First off where are you rpms now? Then, if reasonable, aka within the recommended rpm range at least, time to go through your fuel system. A quick test to determine it's fuel is to take a can of Aerosol Sea Foam with you next time you are out. Remove the cowling and any plastic coverings (air shroud) over the carburetor intakes so that you can look directly into them. Go through your routine and the minute it tries to die, give your carb(s) a shot of SF. If the engine immediately responds with an RPM increase you aren't getting enough fuel.
One of your problems may be pieces of fuel line getting into the fuel channel and winding up in the carb(s) blocking your high speed jet for one thing. On my last engine, at 10 years old, running E10 I had carb problems and found such in 2 of my 3 carbs. One piece was wedged down in the high speed jet which is at the bottom of the carb. bowl and I didn't see it till I applied compressed air, after thorough cleaning and dislodged it.......it was my main issue.
New internal fuel lines and fuel filter (get the flow arrow in the right direction), new fuel line from that tank to the engine, new Merc. OEM squeeze bulb, flushing the tank and strainer in the input of the fuel line in the tank, new fuel pump diaphragm.....anything rubber in the fuel system change it. At 1998 you are wayyyyyy overdue for new rubber. My engine was a 2002 and the problem occurred in the 2012 time line.