Was this after a little time running, like after the first 4 mins or so? If so, then you are a victim of the classic 'half-tank-lean' condition. Basically, as the fuel level drops, so does back pressure, causing the truck to run too lean for the second half of the tank. There are various measures you can take to solve this, and you'll find a lot of info in the HPI monster truck forum about it.

PS. I'm sure you realise this, but 310 won't have hurt your engine if you just do it once or twice, but try not to make a habit of it Mine used to run at 290-300 on an optimal tune and never had any problems. The stock engines tend to run a bit hot.