As a BTW to the starting problem, I have noticed that if you remove a replace
spark plugs a couple of times, the little gasket on the plug gets squished.
I inspected and reinstalled Bosch 3 conductor plugs on my 5000 and found 10k mi
later that the area around the plug holes were getting real dirty. There was
no noticeble effect on performance or economy, but it does seem reasonable that
without the give of the gasket it would be easier to overtighten the plugs and
possibly damage the threads (shudder).
Anyway, with some kinds of gaskets (the springy folded metal kind) you may be
able to restore the gasket before reinstallation. If not, I'd pay extra
attention during tightening.