Originally Posted by jbrians
I even hate to mention something this simple...Have you checked your front QR for tightness? It may be fine on smooth roads but when you are really cranking the wheel could be rocking in the drop outs (blessed are lawyer lips)
Beat me to it. check the rear also. OP there are a lot of things that could "cause" your description including the rider. Take your bike to a good LBS and explain the problem. It should not be real expensive and you might be surprised that they may identify something minor.