Sniady,
Might want to make sure you add a relay from the ignition switch to the solenoid.
In fact you might want to add the realy first to see if it solves your problem.
I think the ignition switch gets corroded over time, and does not provide enough amperage(current) to the solenoid to engage the starter. I had the same problem with my 78 spider, and this solved my problem. Also you might want to lube the solenoid to make sure it moves smothely/freely.
Here is a really nice write up from the Kansas City chapter of AROC:
http://www.geocities.com/MotorCity/D.../spicrela.html