Do you think theres a way to motorize the BIO-wheel rotation? I was thinking about one or maybe two small motors spinning the wheel... Would it really help that much? Would it be worth the time?
Do you have any sponges or filter pads on your intake? I have 2 bio wheel filters and it sounds like you may just not be pulling in enough water to spin it correctly. Both mine have a nice steady spin at all times.
Like others have suggested, make sure there is nothing hindering the wheel from spinning freely. Make sure your getting enough water flow. Sponges or what not over the intakes will slow the rate it can pull in water.
I also have a biowheel and it changes speed... as long as it is spinning it is working. I have sponge material over my intake tube and it does not affect the water flow. Occasionally I have to clean the little things sticking out on the sides when it slows too much.