weird overheating problem

[NotchJohnson]

First of all 50/50 coolant/water ratio is way to high. I wouldnt use any more than 30% coolant, I would actually use about 20-25. Here in fl I use 10% coolant. Coolant will retain the heat more so than the water will. Try reducing your coolant ratio a little bit, and also check the thermo. Check for any leaks also, as the cooling system will only work correctly if it is pressurized.