Jumpy Rpms

Hopefully someone can helpout. I'm experiencing problems with a 95 accord v6 roughly 70,000 miles. The tach gauge has been jumping from about 1000 rpms to 3000 on it's own without reving the engine, nor does the engine rev. When driving the guage will continue to fluctuate from from let's say 2500 rpms to 5000 rpms the only thing you can feel is a slight stumble from the engine. It's been brought in to honda several times and they can't figure it out... any ideas?

recent repairs: patch on overflow tank, and an emissions sensor. both stopped the fluctuation for about a day or two before it came back again.

My 91 Accord did that...Had it at Honda like 5 times and they could never figure it out. Tried a complete tune up new coil and ignigtor and nothing helped.

About a month after Honda had it last the timing belt broke,put a new timing belt on it and it has never happend since.