The stock lenses are good. It probably needs a CLA, which is par for the course with a used lens. Send the lens/shutter assembly away with a check, receive it back with a good-as-new working shutter.

The lens might have a different spring for the higher speeds, so you might notice the higher speeds are more accurate and the lower speeds less accurate until the cleaning is done.

The tri-x should be a forgiving film for this shutter problem. Have fun. I went through the same process last year.