One key issue - are you seeing good focus in the viewfinder, but not on the film? With these old cameras, the foam parts are usually crumbled away. The piece of foam beneath the mirror is a key part - if something prevents the mirror tray from resting on the metal studs, you'll never get good focus. Might check for foam crap in the main camera box that could be clogging things up, or if the camera has been re-foamed, make sure the mirror makes a solid metallic "clank" when lifted (with a fingertip through the lens opening) and dropped.
Though it sounds like your issue may have been mirror slap, if mirror-up has solved your problem.
I had a focus issue using strobes - realized the shutter was stuck open and the modeling lights were giving me a ghost image as I cocked the camera.