Over heating at cruise usually indicates not enough rad or coolant flow problem. Does it stay on the stat ( if you are running one) at idle? You can often "burp" the upper hose while filling the system and get rid of about 90% of the air - doubt that's your problem though. If it's a cross flow rad, you should be able to see lots of fluid movement with the cap off once the stat opens. Temp sender should be near the stat housing. Put a temp gauge in the cap opening and see how it compares with the regular temp gauge.