[GFXjamie]

i would put factory tune back into car for starters.

are your fans turning on at all? if not then theres an issue right there. the temps they engage at in the tuning could be to high not allowing car to cool down when its hot out.

this happened with an old car of mine when i chipped it, it didnt have any programming in it for the cooling fans and when i would be in traffic, line ups, etc it would start to over heat because no cooling going on.

thats what i would check. im sure its something so painfully obvious and simple. could it also be a thermostat problem? maybe its sticking closed sometimes? just throwing some ideas out there. i would definitely check the cooling system/tuning just in case.