Durango overheating question.

First thanks to everybody for helping out.!

Now for the update. Well the cap tested bad. The coolant return valve was defective. Its been a few days and about 500 miles since replacing the cap. Temp runs right where it should. $8 fix.

As a side note it has been to two "higher end" shops over the past six months to try and fix this. Both times it died in the city on the girls and getting it home to me was not really an option. One said it was a blockage in the rear heater core (?) and wanted $1500 it replace the core. The other said water pump at $800. Both times I refilled the coolant and drove it home.