SWF: Needle position does not vary

I have had this SWF machine (1 head, 15 needle, model 1501 i think) for about 2 years now. It has done this on and off the entire time. But here recently we have had it happen more and more.

Here's the deal:
We try to go to needle 14 let's say. When it gets close it makes this awful grinding noise and then starts beeping with the message: Needle Position Does Not Vary. If we punch in another number it will sometimes go to that needle and the message and beeping go off.

Sometimes it moves and keeps beeping. Then at other times it will go all the way over to needle 1 and make the same grinding noise again, causing us to have to power down the machine and let it "rest" as my wife calls it.

Now if we are the only ones with this problem, then we may just need to have someone come out and look at it. If not, how do we fix it?

Thanks for the help!