I have a 55 Chevy

The next time you start your car wait until the temp gage starts to go up past the normal level and turn your heat on full blast. if the temp starts to go down and stayes there while driving than you need to get a thermaostat that opens well before the one you have. also check to make sure you have filled you raditor and overflow tank but please remember that you can only fill your over flow tank to a certain spot when the engine is cold(before you start it) and when it is hot (when it has been running).
If this does not fix the problem please let me know and i will assist you in another direction.