For some reason only in the bathroom the hot water pressure starts okay but quickly drops down but it does happen all the time. The hot water tank is downstairs in the garage. Any ideas on what the problem is?

Sounds like you have mineral that have built up in the heater has got in the system and is now clogging up the hot water supply. Shut the water off at the heater, disconnect the hot water supplies and flush them out. And if you don't flush your heater out on a regular basis let me show you how.
For long life and fewer troubles you should keep your heater clear of mineral build-up by flushing on a regular schedule. Attach a hose to the boiler drain at the bottom of the tank. With the pressure on, open the boiler drain and let it run until the water runs clear. You will see a spurt of red,(rust) followed by white or yellow grains,(lime or calcium carbonate). This shouldn't take more then a few minutes. Do this monthly to keep it clear. Now flush out your hot water lines on ALL fixtures that are affected . Now pull each aerator and clean the screens. Be sure you put them back togather the same way you took them out. Don't forget to flush it out every month. Your heater will thank you for it. Hope this helps, Tom