Sounds like a minor fouling problem, possibly combined with temperature/humidity affecting your mix.
Why a #45 jet ? #51 is stock for most Tomos at 50cc - and I would think one should jet a bit richer when going with a larger cylinder...prolly around #56-#58.
I woud try a richer main jet, with a lower ratio mix...see if you can get more gas in there, with slightly less oil, see if that burns any better...and it should burn cooler, in cause this is a heat-related problem.
That's just educated guesswork, tho...take as ya will.