Kim, I have a suggestion.
It may be that your valve is just fine but that your pouch has 'unhooked' or fallen off of the wall. Either totally or partially.
Your symptoms are awfully familiar.
What happens is that a section of the pouch slips, tilts or slides off of the wall. When your pouch is empty or you are laying on your side, no problem. But as soon as you eat, it fills up, pulls your pouch down from the weight and then pops open your valve. Gas and stool escapes so you run and empty. It gets lighter so it no longer leaks.
You need an opacification (contrast study) of the pouch/valve but with a very specific method: when they inject the Opac into your pouch, have them first drip a few drops into the valve and take a shot to visualize the valve alone, then slowly fill the pouch, remove the cath and take pictures of you on your back, on your right then left side, sitting and standing up. Then do the same after they empty half out.
It allows them to see the position of your pouch when 1/2 empty and full.
That was the only way that we figured it out for me. It also saved me major surgery because that is an easy fix. My French surgeon who does not do K pouches (only J pouches through laparoscopy) but has assisted during one of my fixes, went in through a keyhole and reattached the pouch without opening me up. Miracle. I got my continence back.