I just thought I'd chime in. A few years back, I installed an aftermarket split prism into my old 20D, without realising it, I'd managed to knock out the shim that holds it in by maybe a 1/4 of a mm, which caused a similar problem.
Have you checked that you don't have something isn't preventing it from moving when holding in sideways?
Otherwise it's unusual, and I'd be with everybody else to assume it's shutter failure. Have seen a few people mention similar issues, but then shutters are only good for their max life under laboratory conditions anyway.
From memory, there's a soft shutter function (also drops to 3fps) in one of the CF menus, which you might like to try... Which will at least cover you for the rest of your time away.