if it just started this then it's likely one (or maybe a combination) of a couple things
first, you likely need to retune the engine
just reset the carb needles to factory settings and retune
there's a sticky at the top of this forum that explains how

second you may have a weak glow plug
try a new one

and i highly recommend that you do all this yourself
taking it to the hobby shop does not help you learn the hobby at all