I have a 1994 Geo prism sedan 136,980 mi. I have an overheating problem that we can't figure out. It only overheats in the summer. It only overheats while driving in town, usually if driving under 20 mph. This is what we have replaced so far: radiator, hoses, thermastat, tempature relay, fan motor, and checked wiring. Any ideas what else it could be?