Sounds like a switch or a weak motor. How did you check the switches? Make sure you check them with current flowing through them and not just ohm them out. They will break down when you apply them and the motor wont get enough current flow through the switch to go up. Use a current clamp at the motor leads and see how much your getting on up. Old switches is my guess. You can also use a battery to reverse polarity at the motor leads bypassing all switches to be sure the motor isn't bad in one direction and that's highly unlikely but it does happen. I had a 73 convertible in 79 and I sure do miss that ride!