If it is not an electrical problem, then it is most likely the thrust washers. This model uses two hardened washers that strattle the clutch basket and eventualy they get groves worn into them and the clutch plates no longer septerate as they should. They can be flipped over one time. I used to have to get into the clutch of my DR every 10,000 for this problem. There's a German guy(Ludwig?) who does a nice write-up about this on his Dr350SE site, Google it.
B.T.W. The rectifier/regulator ASM is a componate of the charging system and has no direct effect on ignition(spark).
Playas del Coco
"..an attaching bike on which one rolls serenly along."
(Google translation of a French R100GS review)