I can remember something similar to this happening to my '78 power wagon years ago,
although I could run a wire from the starter relay/solenoid to the + side of coil and it would stay running, then I knew to replace the ballast, however not so easy fix with problem the Ramchrger is having, The RC has electronic dist. Not points as the PowerWagon had,
..... I had hope it not be the ignition switch and if even so? why only after 20-30 minutes should Ignition switch stop working?.
but I suppose that is all is left, there is one other part that I didn't change, I am not sure of its purpose?
looks like a small ballast sets along side of the coil & condiser, could this cause a problem as well? here is a picture of this part.