I confirm this too myself. In 6.22, it was very constant. In 6.31 and 6.4, this is happening a lot. In my 2D game, I watch the F11 debug panel slowly dropping toward 63 then suddenly changing to 66 within one frame then dropping back down to 63. From using a panel to display the value of this:

game_fps = pow(time, -1)*16;

then displaying the game fps value, I can see this value change from around the 64 area then, for one frame, changes to 128, then returns to normal and this change occurs 2 to 4 times a second and would explain the behavior I see in the F11 debug panel. Do you get this in the 6.22 update and does it start occurring since the 6.31 update?