1987 Mercedes 300TD, stopped shifting into 4th gear. Today, I was driving 65mph, the RPM increased rapidly, and 4th gear stopped working. I put it into neutral, then into gear, 4th worked...then after a short distance it happened again. Three days ago, I replaced transmission fluid and put 2 magnets into oil pan since there were none by the drain plug area. Magnets were household ones that I found in my garage. Could this be a problem? May be vacuum problem? I checked the dipstick and for warm engine, it shows overflow. Can this be the cause? Thank you.