I guess this could have been part of the issue, I've since reset it, so hopefully ok now
I had run the motor in for a good few minutes running it slowly using the throttle trim as suggested by others on here.
I was running on concrete for the most part, with a little driving over grass verges.
I've seen this sort of thing mentioned a good few times re 15t motors (including in this thread!)
True, this is only really a monster truck in name, it's far more like a stadium truck in reality.
I wasn't so much blaming the Lipo as just having been disappointed with the results. I've been running the 18t motor with the Lipo, and although it's getting hotter than it did in my other car with the nimh it is certainly lively!!
I'm guessing your gearing must have quite an influence on the temps though I'm struggling to get my head round gearing really.
I'm going to keep an eye on the temps of my 18t while using the Lipo and go from there I think. Depending on what I see I might have to look at changing the gearing a little though that confuses me a bit as the more I've read the more confusing it's all become...lol