have you checked any of the bits that wear (i.e. needle - look for the needle being ovalised, sorry, ovalized). It is unlikely that a main jet would change size so a good clean should fix it (carb cleaner). Clogged air filter could also cause the same symptoms - enough air gets through for low consumption, not enough to sustain high revs.
What carb does the bike have? A buggered accelerator pump as fitted to some models could also cause the bike to have problems during sudden throttle opening. But this would be short-lived
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