I'd had 'strange' tach issues for two years, where the tach would briefly go to 0 and the engine would run sluggish, then return to normal along with the tach. About 3 weeks back, the tach went to 0 and stayed there and the engine was clearly running on a single cylinder. This lasted for about a minute then the engine ran normally. Same thing happened the next two times I rode the bike. I swapped in a spare TCI, which replaced the problem with similar ones.
I purchased a non-programmable TCI from Ingnitech, installed it, and have had no problems since. Performance is the same as it was with the OEM units.
I think my 40 year old TCI and similarly old backup are past their useful ives. But the new one seems to be working great.