Next time it does that (with a full gas tank), pull over, take off the gas cap for a second, put it back on, take off, if it doesn't lose power for a few miles it's probably your gas cap is not venting or you have a loop between your petcock and your carb.
If it still loses speed right after taking off and replacing the gas cap, I can bet you a dollar is you larger jet.
Before you dig in the carb make sure your air filter is clean and try seafoam @ 1 oz per gallon of gas. The other thing you might try is to "starve" the bike. (With the bike parked and idling, turn the petcock OFF give it a bit of gas 2000 rpms or so, when the engine I'd dying give it lots of gas 5000 rpms or so) that sometimes pulls particles thru.
But if you want a clear shot, pull out your carb, clean put it back together and go for a ride.
Every ride is a joyride on a Rebel
1986 CMX250C "Miss Triplehorn" 17K miles
1984 GL1200A 111K miles