Are you meaning to say that you get a slight weave at a certain speed?

If that is the case you may want to check that your pressure on the front tyre does not exceed 28psi and also check if the tyre itself is beeded properly either side of the rim because even the slightest chink in the bead might be the cause of the problem.

If all fails then I would be checking your steering head bearings, in particular the one in the lower part of the fork just below the head tube because usually road bumps and constant vibration are causing wear and tear of these bearings.