I had a similar problem on my '86.
The rotor was not seated all the way down onto the shaft. The result was that the carbon contact wore down, and eventually seized into the cap, the finger that locates the rotor on the shaft was worn as you describe.
This is easy to do on the Carrera because the rotor is a snug fit on the shaft. It can feel like the rotor is on all the way, even though it is not fully seated.
Another telltale sign is to look at the electrodes in the cap. See where they are wearing/arcing. If it riding up way high...then your rotor is not pressed all the way down.
My car ran fine this way for many miles. Then started to run like crap/not at all.
The solution is to get a new cap and rotor, and be damn sure you press that rotor all the way down. Give it a little twist while installing until you feel the finger start to catch, then push it down.
If you suspect it is not an install problem, but instead that it is working its way up, then install it correctly, drive and check every so many miles to see if it is in fact working its way up. Don't remove the rotor to check.
Measure it's height using a ruler, relative to the distributor body.
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Chuck Moreland '86 Cab - "Sparky", '77 Targa - "Sweet Pea"
[This message has been edited by Clark Griswald (edited 06-26-2001).]