We just bought our first Mustang, but not the last. A '99 V6 with 94K miles. After running for a while, fifth gear grinds on engagement.  This doesn't happen going into any other gear and double-clutching prevents the grinding, so we think it's a problem with the synchronizer.  Has anyone run into this problem before and if so, how did you go about fixing it?  If nobody has experienced this before, what do you think the problem is and how should we go about fixing it?

Once again, the up-shift from 4th to 5th begins to grind after 3-5 shifts into 5th or about 10-20 miles(before that it works fine) and no other gear does this.  Is anyone thinks they needs more information to "diagnose" the problem, just ask.
Thanks in advance  

It does sound like you are loosing your synchro. I'd look at either having it replaced or just swapping in another T5 depending on your budget.  

I had a similar issue.My fifth gear started grinding as well.After two years of Hard driving the car the World clas T-5 started slamming out of second gear under heavy loads and rpm's.And fifth became even worse.I do suggest you get your's rebuilt if not bullit proof it.