The electric tachs are far more steady than the earlier mechanical versions. You may simply have a bad electrical connection.

There are only 2 connections for the electric tach:
• One 1/4 spade connector on the RH ignition coil. That's where the signal originates and travels on a single wire.
• Then there is the multi-pin connector on the back of the instrument cluster. That big rubber block connector. Often times the single hold-down screw has been over tightened and the plastic is stripped out, leaving no way to cinch up the connector.

I'd start with an application of No-Ox-Id to eradicate any corrosion that may be affecting either of those 2 connectors. Both those connectors see a lot of water, so this is quite possibly your only problem.

Thanks for those suggestions. When the fuel falls to a easily manageable level, I'll pull the tank and check the coil connection. The last time I had the rubber connector off the back of the instrument assembly I though I sprayed contact cleaner on the pins, but that's an easy re-do, so I'll start there first. Am I correct in thinking there is nothing that may require a half drop of machine oil in the instrument?
One of the puzzles related to this problem is the fact that it occurs only after sustained high speed riding.
It was an issue going to and returning from the 49er Rally and the Chief Joseph Rally this year. Around town it never happens.

Thanks for those suggestions. When the fuel falls to a easily manageable level, I'll pull the tank and check the coil connection. The last time I had the rubber connector off the back of the instrument assembly I though I sprayed contact cleaner on the pins, but that's an easy re-do, so I'll start there first. Am I correct in thinking there is nothing that may require a half drop of machine oil in the instrument? One of the puzzles related to this problem is the fact that it occurs only after sustained high speed riding. It was an issue going to and returning from the 49er Rally and the Chief Joseph Rally this year. Around town it never happens.

It does indeed present itself as a mystery. I always attach these type issues with "regular service steps". (By regular I mean reasonable, as in what you would do at a 100,000 mile tip-to-tale service.) During said time of scrutiny, the offending culprit usually reveals itself. Typically the bike is simply saying "spend some time with me."

► You absolutely DO NOT want to lubricate a tach or speedo with regular mineral oil, or you will be in big trouble when that oxidizes in 6 months. If lubrication of the movement is required, then you want to use only silicone watch oil. And you'll apply 1/2 drop to each pivot point with a dental pick or watch oil applicator.

► Really the best thing that might come out of gauge disassembly is the chance to inspect the very fragile flexible circuit board inside the gauge pack. People have been piecing those together for years now until they reach the state of full-fledged "bodge". Thankfully, new circuit boards are now available.

► The fact that the needle swings easily and points correctly for the first hour indicates the mechanism may be alright. Erratic behavior might then set it with a connection that loosens with heat or vibration, at least that's my theory. Remember the gauges are rubber mounted and move quite a lot.

► You might also take a dental probe and slightly close up each of the C-shaped socket contacts inside the big block contact on the rear of the gauge set. It won't take much at all.