There is a good possibility that the oil seal is leaking and allowing trans oil to get into the solenoid. This will cause chattering and eventually cause it to stop working.
You can dissassemble the solenoid and wash it out with cleaning solvent. Let it hang upside down for a couple of hours to dry it out. As mentioned previously clean up the contacts. If the contacts are really worn they can be built up with silver solder.
The oil seal P/N is 3060CR. Napa or a regular parts store should be able to get it for you.
Fred
