without pressure being held in the radiator, the engine will definitely overheat during normal driving. JD is right, first fix that and then chase down the other demons. BTW, I have blown head gaskets in the past, one of the best tests you can do is take it down to your local auto shop (any will do) and have them test for the presence of oil in the coolant. In my case, I had no white smoke in the exhaust, no foam or any other indicators in the coolant, but the oil test ran positive and I rebuilt my top end that same weekend.
Slow starting can be caused by a ton of things, is your engine cranking slowly or is it just not firing right away?