Camera, as others have outlined. If the film is touching other film on the spool, you'll get big purplish/blueish spots that go outside the frame. Here's an extreme example:
Film edges and the writing on it are good indicators of problems. If the edge writing looks fine and the image problems are limited to the photo frame, it's usually a problem with the camera mechanism (ie shutter). If the edge writing is affected, or dark spots extend to the sprockets, that indicates a light leak or a problem that happened outside of the camera.
I think someone gave you the tip of pushing the film into the reel and seeing if it moves or resists -- that's a great way to tell if the film is stuck. I also recommend doing some winding in the light (with a scrap roll of film) to get a feel for it. I practiced with a "burner" roll of film before I got back into developing.
As for the repairs to your MX: If you had the shutter fixed two years ago, it should not be giving you problems. A properly CLAd Pentax should go a couple decades. MXs are wonders of miniaturization and really do require specialist repair. Send it to Eric at pentaxs.com, his prices are very reasonable and he knows how to repair these fiddly cameras. I own several old Pentaxes and I think there are a few tell-tales that they have been repaired by someone who wasn't specially trained to fix them.
Aaron