I have the same type of problem, been that way for the last few years. Except mine does it past anything above 2500/3k rpms. The needle will go up, then bounce back- it will even go below the 0. (annoying) Eventually I will get to taking the gauges apart and checking it out but I just got a autometer tach. When I posted on this before people said to just disconnect the stock tach wire, etc. I use to have msd and they said that could have caused the stock one to go bad?? Let me know if you guys figure this out I really want to get it fixed.
It's likely a signal strength problem. cevtv's suggestion may work if there is a poor connection somewhere. If it doesn't, then you will likely need a signal amp of some kind or will have to delete one of the two units you're attempting to run from the signal feed.
Other things you might try are direct wiring your RPM dependant devices to your coil signal wire, or perhaps even swapping in a different coil to see if there's a change.
You might also want to check for corrosion on the connectors from your ignition to your coil. You can have a signal strong enough to trigger the coil but weak enough to cause other problems.
Where is the coil signal wire? I have the shift light wired to the coil ground wire. It works when it wants to, and it has a solid connection. I don't get it!