Mansfield 160 toilet that spewes water

I have a four year old Mansfield 160 toilet that spewes water out from under the tank lid. It only amounts to about a pint of water, and it only happens about every other week or so. The toilet is used several times a day and never spews water when I have the lid removed from the tank. I have installed a new valve assembly and the problem continues to happen. The only thing that enters my mind that it may be possible that air is some how getting into the city water system and might be causing the problem. However, I have never noticed the problem with the other Mansifield toilet in another bathroom. Nor have we expereinced air from any of our faucets. Do you have any suggenstions?
Keilnult

In a typical toilet, the handle is higher than the overflow tube, so even if the valve runs continuously, water won't reach the point in the tank where it can escape to the outside.

The only other points where water can escape are the tank-to-bowl bolts or the gasket between the bowl and the tank.

The case where you usually see this is where the refill tube has come loose from the clip, but if this were the case, it would not just be every week or so, it would be every flush. But anyway, look at the refill tube

On the mansfield toilets the refill tube plugs into the center of the flush valve.