My 96 me overheats sometimes after i have been driving for about 30 minutes. It seems to only happen when i am cruising in 4th gear or when i am sitting at a stoplight. The temp gauge almost gets to the 3/4 mark but never goes beyond this. It seems to reach this point and then goes back to normal. What could this be? I took it to the dealer but they couldn't duplicate it. I am hurting my engine by driving this.