Mike,
It still sounds like a chain issue to me. You said your chain tension is correct, 1/2 to 3/4" So let's look elsewhere. I just posted this reply to another member who is having the same problem but worse than yours.
There is a point inside the clutch actuator cover where the chain hits. It could be yours has even less clearace there than some. Here's a link that might help you. >
http://motorbicycling.com/f4/chain-noise-6682.html
The other thing that might cause the chain to grab while parked is rough teeth on the engine drive sprocket. I've never encountered this particular problem but it has been reported on enough for me to believe it is a possibility. The fix seems to be to smooth the teeth a little with a Dremel tool and the appropriate grinding stone or sanding drum.
The only other thing I can think of at the moment is binding rollers on the chain. Some of the kit supplied #415 chains have tight or stuck rollers that can cause a bind when going over the sprocket, especially the small engine sprocket. Hope one of these clears up your problem.
Tom