The quick release part of my cartier roadster strap keeps undoing itself

YOU MUST TAKE YOUR WATCH TO A CARTIER STORE !! Especially if it's till under warranty !! If you or any 'regular' watch repair shop opens that watch you will destroy it's value, and they CAN tell if a non-Cartoier tech has opened it. A regular watch opening tool will scar the back of a Cartier. Normally a tune-up is $60, but you say your watch is still warrantied, no reason not to take it to Cartier. You can Google Cartier stores to find one closest to you, get their number & call them and they'll advise you further. I urge you NOT to they & open that watch yourself ! You're only looking at a few dollars to insure & ship it, just make sure you wrap it very carefully, lots & lots of bubble wrap. A tip: wrap the watch in several layers of bubble wrap/tape snugly, then use a clean hand towel that you snugly wrap clear shipping tape over top of, it'll provide alot of safety for your valuable timepiece. You must insure it, don't skimp on that, you could end up regretting it bitterly. Hope this helps you~