[east10ac]

I just bought a 2001 neon 2.0 automatic,the problem is it only overheats when your driving it at hwy speeds, I can drive it around anywhere in town and it doesn't overheat but when I get it out on the road say around 45-55 it starts to overheat fast.
I'm an old school mechanic pre 1990 but have a terrible time with this neon

things I've checked or done

changed thermostat
ckecked for good water circulation ( good)
pulled the cat of and it looks to be almost new! not clogged at all (maybe someone had the same prob)
good even compression
no water in oil

this thing has got me puzzled

something else I noticed the harmonic balancer is wobbly and needs rreplaced.

last minute thought! ( vaper lock in the cooling or collapsing hose? )
any ideas ?
thanks, Allen