When my car is cold it drives fine, no problems at all but, When its warmed up it start's to hesitate and lose power during acceleration usually around 2000-3500rpm, It's feels like it's misfiring, once I accelerate to about 4000rpm it stops and drives like normal.

It's only been doing this ever since my muffler fell off, I've been driving with no muffler for about 2 weeks and it's gotten worse. I'm having a new catback fitted next week but, I'm not sure that will fix the problem.

I'm thinking it's something to do with my Lambda/O2 Sensor, maybe related to me having no muffler, might that be what's causing it?