I recently started having cooling issues with my GS300 that I purchased at the beginning of the summer and I was hoping to get some advice. The radiator appears to be pretty new.( no more than 2 years old) It's an aftermarket one made by CSF. the car started running warm (not overheating gauge never got close to the red) one day and I found the water pump leaking so I had it replaced with an O.E.M. pump along with the timing belt, crank seal, and both cam seals. I checked the coolant levels every day I noticed it was blowing coolant out of the reservoir overflow after I drove on the highway but, the car was not running hot so, I just topped the radiator off. Then one day after about a 90 mile trip on the highway the car started to run hot in traffic within 5 minutes after I exited the highway. I turned the heat on and was luckily able to get off the road. I observed both fans were running at that time and appear to cycle on and off properly. I was worried about a head gasket issue so I had a block test done and it came back negative. Also had cooling system re pressure checked just to be safe no leaks so, I replaced the thermostat with an O.E.M. one thinking that was the issue but, still the same thing blowing coolant out of the reservoir overflow tube on the highway never around town, even in stop and go traffic everything is fine. I replaced the 13psi radiator cap with another CSF 13psi cap hoping that would solve the problem but no luck. I think the head gasket is blown because everything has been replaced with O.E.M parts except the radiator and all appears to be in good working order. It only seems to happen on the highway and not around town but, I can't explain how it would pass more than one block test and coolant pressure test. The oil looks good. Nothing on the dipstick or under the cap. The car doesn't smoke and runs great. No engine codes. Any thoughts on what could be the issue?! Need help!!!