I have a two tap waltec set for a shower and had trouble with the hot water dripping when we turned off the shower. I replaced everything - cartridge and seats and now it doesn't drip any more but I have an even stranger problem. When I turn it on as soon as the water gets hot the hot water shuts right down. If I turn it back on it only works if it is quite wide open which wastes a lot of water. What the heck is going on??

Disassemble the hot tap. Place a cloth over the open tap and turn on the water briefly to flush out the tap. You said cartridge and seats. If you meant the older style compression stem with washers and seats, make sure the washer is tight. If it is truly a washer less cartridge, the hot and cold cartridges should be interchangeable. Move hot to cold and see what happens. If problem moves to cold you have a defective cartridge.