Lots of good tips here. I don't have anything new to add, but I agree that it might be the gears. Look in the gears and see if there are any hard particles that are jamming them up. You may have to clean the gears and replace with fresh lubricant. The gears may be located in a couple of places...below the machine, or up in the arm.
I also second throwing away any old oil you might have and buying new. It does lose its viscosity over time. Check your manual to see what kind of oil they recommend.
The loose belt idea was a good one and also a possibility.
~ C