Most common problem I've come across with these is the brake arm to the right side of the take-up reel table. I think this may well cause the symptoms you described.
There's a thin lever with a tiny felt brake pad at it's end (look to right side of mech viewed from the front).
It nearly always peels off or falls off completely causing the lever stick or jam.
It's unlikely you'll be able to get a replacement anymore, so look for a tiny black pad in the mechanism and reattach with the smallest amount of super glue gel to ensure no glue leaks through the pad.
Generally on an older machine, belts will be slack and break pads worn.
Other problems, check where capstan belt attaches to capstan flywheel, sometimes the press on pulley wheel splits and either causes belt slippage or pulley falls off completely.
Cobweb, October 2005