Temperature problems after switching to RAMPS 1.4
For the overheating, you may need to turn the juice down a little on your Power Supply. Typically, there is a potentiometer inside that trims the voltage level coming from there. Some things can heat up if it's over-volting and needing to drop more voltage for the board.

For the temperature reading stuff, make sure the thermistor is plugged in all way and not possibly dangling near the hot-end block. I have had that happen before and it will report improper temperatures as it goes along, and often results in the hot-end getting too hot and messing with the filament. If it's properly secured it might be just crappy or broken in some other way and you would need to replace it. It's reasonable practice to have a few extra thermistors lying around as having them go out at lousy times will bum you out.