Instead of assuming, scope the grids and SEE if they are going positive.
And if the grids were drawing current, what is the current rating of your FETs? 100ma of grid current ought not to overtax a 5 amp FET.
I am not up to speed on MOSFETs, any chance yuor problem is one of polarity shifts? That is if the grid rises to a point where the MOSFET sees it as a reverse polarity voltage somewhere it doesn't want to see one?