I have a 1996 GMC K3500 with 6.5 turbo diesel 4 X 4 that is over heating when under load, it runs good an flat ground no problems what so ever, but as soon as I go up hills with ot with out a trailer behind it, it runs up to 220-40 fast, almost having to back all the way out of it and stop to get it to cool back down to 200, normal runs at 200. what is the problem?