starter

Obvious first step is to chech your battery and its connections. Make sure they are clean ect... I'm geussing you've done that though so.... My guess would be your ground wire is worn out... If you want to properly diagnosis such a thing you can hook-up and ohm meter with the battery disconnect and look for abnormally high resistances. Or since a ground wire is fairly cheap, you can just replace it. Not sure of the electrical system on your particular car but many times you may have a couple to replace... A chassis to the negative terminal on your battery and cable to whatever grounds your starter. I hate to recommend it but I'd just start replacing the wires that look worst... As old as your car is, it prolley has some worn out main wires. Won't affect the things that don't draw high amperage but your starter might not be getting the amperage to turn over properly. Since it's a 70s Eurpean design it might have what called a ground strap from the chassis to the engine or the tranny... They can be hard to find an OEM style one but any battery cable of sufficient gauge that you can hook up in it's place ought to work.