The DoF preview and the mechanical release are both parts of the mechanical mirror mechanism, so something probably needs to be fixed in there; the mechanical release is just a cam with a lever that trips the mirror and then fires the shutter at one mechanically governed speed; the DoF preview lever/lockup is also linked to the main mirror lever, so perhaps something is amiss with the actuating levers.

My concern would be that it is firing at 1/60 all the time.