whey, another DT owner
i've had that problem before, first time it was the jets needed changing up a size and the second my carb was loose and leaking air. so:
1: check your fuel system for loose fittings, leaks, perished seals e.t.c
2: run it up and down the road a bit at full throttle, then cut the engine and coast to a stop, take the spark plug out and check what colour it is on the end (should be a chocolatey brown, but i suspect it will be white and chalky indicating a lean run)
3: take the carb bowl off and tell us what size the main jet is.
also, list any other engine mods you have, the age of the bike, its mileage and preferably a bit about its history, most likely case is that the jets are not big enough causing it to run lean, heat up and ceise
these bikes are extremely hardy and it takes a lot to kill them, i've made some mistakes with mine in the past and ceised it up, blown a big bore kit e.t.c, but with the help of the Yamaha Owners Club i have managed to fix most problems quite quickly