I have a '99 Silverado 1500 2WD, 4.3 Vortec. My tachometer will just drop to zero or be at zero when I Start the engine. It will also start back up randomly. It can cut in and out several times on my typical 30 minute drive to and from work or cut out and not kick back in or just be normal for the entire drive. It has cut out just coasting down hill on a newly paved road, so I do not believe it to be a loose wire as it does not necessarily do it on bumpy roads and I do live in the country. There is no noticeable performance issues, however when the tach is at zero and I'm cruising at an even speed, if I let up on the throttle there is a lurch. This even happens on cruise control such as when going over an upgrade. Not drastically, but more noticeable at slower speeds. Oddly, it only does it on my evening drives home although I have made the drive with it at zero and no lurch regardless of the time of day. I have tried to recreate it, but the condition has a mind of its own and is completely random. Other than the lurch, it still accelerates and starts just fine. Fuel economy is the same considering it has over 300k miles on it. I thought air temperature might have something to do with it, but as fall has arrived, it can be just as cool in the evenings as it is in the mornings, yet the lurching condition will appear only in the evenings. When it cuts out, if I don't see the needle fall and don't notice it right away, however when it cuts back in I can sometimes detect a slight jump, not like the lurch. I've checked the connections at the transmission and they appear fine. It's got to be electrical but not sure where to look. It does not make sense.
Any suggestions on where to troubleshoot this condition would be greatly appreciated.