Sounds like the stepper motor that runs the tach may be going.
I have been doing alot of reading about gauges doing this and everything I found says stepper motor. There are places that can replace them for you or if you are good with soldering you can get the stepper motors off ebay for like $10 each. If you are going to do it yourself replace all of them and do it just once.
My nephew just got a low mile 05 V6 5 speed. The tach was doing this, then the gas gauge, and then the speedo, the temp hasnt done it yet.
they will read high, read low, stick somewhere in the middle of the range or not move at all.
The dealer he got it from is having a ford dealer replace the cluster next week for him.
I highly recommend you do it yourself or pull the cluster and send it to a place for repair. The V6 cluster is about $400 at the dealer. Labor to program it and install it is $145. damn glad we arent paying for it.
Do a quick search on "stepper motor" and you will find more details.
Here's a nice write-up that I came across a while ago. I need to do it myself since my speedo is dead as well as my coolant temp is on it's way out. Mine's an '05 as well.
If the used car dealer hadn't agreed to pay for a new cluster I had a back up plan. There is a guy on ebay that offers repairs. He replaces them all for like $100. He only lives about 20 miles from me and said he could do it while I waited. It would take a little over an hour he said.
The only think I am not sure about is how any of these shops calibrate the needles.