This is one of the most common problems with focal-plane shutters in older cameras, or if they've sat unused for awhile.
At certain speeds, the opening curtain fails to fully travel across the frame before the second curtain closes the gap. A standard service repair (CLA) from a competent shop should take care of this. Usually it's just a lubrication and/or adjustment of the shutter mechanism.
Fortunately, the older Canon SLR's are robust, well-made, simpler cameras, so you shouldn't have much trouble finding a good repair person.