[RC-Monster Mike]

Marvin,
Try swapping to the 1400Kv motor on 6s Lipo for starters. The 1400Kv motor is the best option for this voltage IMO, and will be generous in regards to runtime. I would surmise that your driving style is also a factor - I can get the same runtime you get on a 4s 6000mah pack - only 3/4 the battery power(and while I am no pro, I don't suck, either). I use a 1900Kv system on 4s geared for around 40mph in my buggy. The 1700Kv on 6s will suck more power, but the 1400Kv on 6s should be pretty close to my setup.
Someone above mentioned to drive it like a 1/10 truck(meaning smooth throttle input) - this will make a big difference vs the constant "pulsing" that is often carried over from nitro drivers. In nitro, pulsing the throttle is helpful to keep the clutch engaged to a certain degree - in electric, it wastes power and upsets the chassis. Dropping this habit will lower mah usage and temps significantly.
So...swap to 5s Lipo or the 1400kv motor and focus on smooth throttle input and you should see a noticeable improvement in runtime(assuming there is no binding, etc. as also mentioned above).