Overheating wowes that make no sense - help!!!

a leaky radiator cap can cause all kinds of heating issues too, seems trivial but the pressure has to be held in the system to keet it from "boiling" ie (filling up your overflow tank)...test or get a new cap, flush the block....then the others advice about a head gasket may be the culprit..i agree the fan isn't it if the cars moving at 50 mph..new rad takes the blame away from it...so just take it a step at a time. good luck!