I found after riding in traffic that after accelerating in 2nd, I could not shift into third gear until a pause with some reduction in RPM and pre-shift applied. Under slight acceleration, shifting is fine - a bit clunky into 3rd, but always shifts. This is a 1978 (08/1977) five speed transmission. I don't remember having this shift problem before now.
The problem happens after accelerating in 1st to ~4k RPM with mid throttle then similarly in second to ~ 4k mid throttle. The bike is stuck in 2nd gear and it takes accelerating in 2nd to ~ 5k RPM (to keep up with traffic) and then reducing acceleration to shift into third. After experiencing this, I instinctively changed the transmission oil but don't expect that will make any difference. This happens when accelerating with traffic on 45 mph roads. Is the flywheel the culprit - and lightening would help, is it my shifting technique (i.e. stop accelerating before trying to shift into 3rd), or a worn gearbox shifting mechanism, or some combination?