It's hard to say what's happening for sure. But I have had similar problems with a 46FX. I changed the glow plug and it made a world of difference. I run 15 % nitro fuel which means I can run a cooler plug most of the time but with certain conditions I need to use a hotter plug. Try one thing after you start your engine. Idle it down. Does it slow down when you remove the glow driver? or Speed up when you reconnect it? That can sometimes be an indicator of a bad plug. If it doesn't do that. Try using a larger or smaller diameter prop. On hot and humid days(mostly humid) I need to go to a larger diameter prop for it to run right. Whatever you do do only one change at a time.
Mikee