50cc dies to 10mph when engine hot

[skyrider]

Have a used Piaggio 50cc 4stroke with 2,600 miles do all my work from excellent Scootdawg instruction and Lefty from years back! Working great for 2 yrs then trans started slipping, so a few weeks ago replaced rollers, plug, found a loose hose to airbox, so it runs great now except that after running hard at 35-40mph for about 5 miles it dies down(like no gas) to 10mph. Then after it cools for 20 min to half hour runs like nothing happened. Used to put Seafoam capful in my Chinese scoot every fillup, but haven't with this yet put some in a few days ago but no change. I did empty the tank and the fuel filter is totally clean, put fresh gas in. Since I got the scooter it always had a rattlin muffler with acceleration and the shop thinks that's probably the problem. They want me to bring it in for $120 to check it out this coming Wed. But I'd like to be able to fix this myself. It seems like something is cutting the gas flow when a certain temp is reached, could it be the inside of the muffler is shot and that's creating a vacuum to restrict the gas flow? Should I take off the muffler and ride and see what happens?