I'm dealing with a 20 year old Kohler commode that periodically overflows the rim. The water level in the tank is fine. When I flush it, the flush starts "normally" but after the bowl nearly drains it starts to re-fill (like at the end of a normal flush), then drains again. This happens 3-4 times before the bowl fills normally. The line has been snaked, I've replaced the valve and flapper, and I've snaked the flush "route" in the china from the bottom up and the top down (after removing the commode from the floor. Any thoughts? I've seen the posts on a commode "wearing out" and do not have any lime buildup.