Engine stumble, erratic rpm reading

I've developed an intermittent engine stumble or miss at highway speeds along with erratic tach readings of 1000-1500rpm above actual rpm. I believe the stumble always occurred when the tach was showing higher than actual rpm. I've been looking for bare wires or a short at the coil. Any ideas? The only other times I've had goofy instrument readings was when the battery or charging system was going bad, but the battery is being charged fully.

Could the tach be going bad?

No, I don't think so, not with the engine stumble occurring at the same time when neither problem existed before. The bike is a '92 FXRS. I've had the same coil and plug wires for about eight years with no problems before. The tach does read correctly part of the time. I'm going to make sure the chrome coil cover isn't touching somewhere and take a test ride.