Half black photos after replacing superimposed display?

I recently had my 5D II's superimposed display replaced at Canon Repair in Irvine, CA. I just started using it again, and every couple of photos, half the screen is black while the other half is exposed.

I'm not using any flash. Any ideas what could be happening? Busted shutter? My wedding is in 7 days and I need to have my 5D

Yeah it sounds like a shutter issue since you're not using the flash. Does it do it at all shutter speeds or just faster ones?

Just the faster ones. Slower ones nothing wrong, but faster (around 1/200 and above) and the problem happens.

Weird. It sounds exactly like what happens when you exceed the max sync speed with flash. Does the black get progressively larger as the shutter speed increases?

Haven't gone that high up in speeds, I'll let you know.

It could be a problem with the reflex mirror not fully retracting. Try some slow exposures with the lens off while you look into the front of the body. In any case, it sounds like it'll need another trip to the service centre.

I had a similar issue back in the day with a Nikkormat FT. The first curtain was bouncing and at fast speeds, the effect was part of the frame unexposed.