Is there any evidence of oil on the shutter leaves?
Take the film back off, and trigger the mirror lock up while peering through the film gate, do you see light as the shutter closes? You should not, if you do, then the shutter leaves are slow (usually due to oil contamination) and they need to be cleaned.
During mirror lock up, the primary leaf shutter does not "open", as it starts out opened (so you can see). It should close, before the light baffle in the body opens. If it does not close in time, it needs a service.