Have you checked the water pump! I've had a few fail which have given rise to the symptoms you describe. Easily checked and replaced.
Regarding the gearbox. Assuming you have the original four speed Ford type, it sounds as though the plastic washers at the bottom of the gear lever are starting to break up. No service kits available so only option is a replacement if you can find one or use an aftermarket rose jointed one. Both of which will need to be shortened and bent to suit.
Arwyn
