Having 4 5050's and counting there are a few things that have worked for me. First get some contact cleaner, ie; Caig DeoxIT. Try spraying the internal Ho-Low speed switch on the control board. That's the one that folds down. Just cleaning that switch has fixed odd brake and speed/operation problems on 2 decks. Next unplug a re-plug all connectors on the boards. Then clean the front panel speed switch as well as the speed switch on the head block. Try working all the switches 20-30 times, including the switches on the back panel. Look at the capacitors on the control board. Inspect those cans for bulging tops amd or leaking goo. See this thread for tweaks:
http://www.tapeproject.com/smf/index.php/topic,2037.0.html Generally if it plugs in pull it and re-seat it. Also the capstan motor should be movable with the deck off and static. If you push down on the capstan it shoud move down easily and spring back up. If it does'nt you need to remove it to clean and oil the assembly. I will be doing this to one of my decks soon. I'll try to document the steps with pix. Enjoy!