I'm sure the bike is fixable, but if the mechanic reckons they all do that, clearly he hasn't a clue. I'd trust the bike but not the dealership. However, you don't know how long it's been doing this, so I'd go and find another one.
This one didn't sell on Ebay a couple of weeks ago, maybe still available, it seemed like a bargain to me, but I wasn't looking for a 750!
http://cgi.ebay.co.uk/ws/eBayISAPI.d...MEWA%3AIT&rd=1