Check the top of the distributor shaft to see if you can shake it side to side. It sounds like the point gap/dwell is changing due to distributor shaft wobble from a worn upper bushing, which is changing the timing. For every degree of dwell change, the timing changes two degrees. At higher speed, the timing will appear to stabilize.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!