I recently rescued from my parents basement my moms old 1985 trek 600 series. The bike is in pristine condition as it has always been stored inside and it hasn't been ridden in at least 10 years, everything on it is original except for the bar tape. I gladly swooped it up on a recent visit back home to MN and brought it with me to Denver. I oiled it up and dusted it off a little and it was good to go. On a recent ride though however I was going to cross the street at a red light and as I put torque on the chain the whole rear wheel jerked right out of the drop out (I almost went down in the middle of the intersection!) I pulled the wheel back into the dropout and snugged it up good and thought I would be fine. About 20 mins later the same scenario happened in a less busy intersection thankfully. I phoned my gf to come pick me up because I was very concerned it was going to happen again and it bit up some of the paint which was kind of a bummer. I took it to Bicycle Village later that day (they were the only shop open at this hour on a sunday) Needless to say the guy in there was a total prick and pretty much tried to tell me my bike was a piece of **** . He was little of no help and basically just re-tightened what I had already done previously. I'm sort of reluctant to ride it at the moment because i'm afraid this will keep happening. It has the original titanium wheels on it but they are straight. Will I need to replace the wheel or is there something wrong internally that I don't know about? I'm looking to get some insight on what this problem might be caused by it is the first real issue i have had with a bike. Thanks Guys!