Have you ever been able to ride thur the wiggle?? Had this happen on one bike around 50mph but smoothed out at 51mph only had to worry about the way back down. like jeff indicated think wire and harmonic resonance it could be that at 32-33mph the wiggle/wave from your wheel is resonating and amplifing thur your frame.

with my cuurent bike with the wheels I built up myself I have gone 50-57mph several times with smooth sailing all the way.