Presuming that this model has a Bosch starter like an E21, the solenoids, mounted right on the starter, have a tendency to get balky when the lubricants inside get dried out from age (kind of like me). I've fixed a lot of these on VWs and various other European cars over the past 30 years, including one E21, just by disassembling them and carefully relubing everything in the solenoid linkage. Someday, however, you're gonna need a rebuilt starter, and unless you can do it yourself while watching TV, the labor in getting the simple relubing done by a real professional may be close to the cost of a rebuilt.