John sounds like you may have a problem with the air correction jet, richen up the top end by popping in a revised diameter insert. I personally found that for the w/cooled engine running a 34mm MK2 with a power jet was a waste of time and the engine ran sweeter with out it, besides, who needs the complication. The electronic system on the Honda RS now that was essential, and allowed for maintaining reasonable power right up to 14,000rpm
The conundrum is that the traditional power jet richens, whilst the electronic weakens!
What temperature are you reading when the alarm bells ring? Too much retard at peak torque can dump a lot of heat into the pipe and raise temps there, but is not necessarily detrimental to the engine, too hot a plug grade doesn't help, lots of possible causes but diagnosis can be tricky. Be interested in how you overcome the problem, keep us posted.
Trevor