chances are you don't have enough spoke count, and aren't keeping your spokes tensioned. I had this issue with a 28 spoke wheel that came factory on the Raleigh Revenio; my solution was to buy a 32 spoke wheel and have the bike shop swap cassettes, and this ended my problem.

When I purchased my Orbea in January it came with a 32 spoke wheelset and I haven't had any trouble keeping it true. I am 360 pounds and 6'3" tall (and losing weight). Buying the single pre-built wheel cost me $199.00 and if you are only having issues with the one wheel I would only change out one wheel.