It could be from the weather changing. As it gets colder it leans out your engine.

 

I would guess its your idle. If you had to turn up your idle to keep it running or if you have to use the choke a lot or blip the throttle to get it started you are lean on idle. Most raptors are lean there to begin with and pipes usually make it worse. Go with a 25 pilot and tweak your screws and you should be fine.