Pardon me for arriving late to the party, but...
When we had that same problem, after several disgusting episodes, I go a really good look at the valve in question and realized that the cable had a kink which made me think that the issue was the valve because of the resistance in trying to open or close the valve.
The valve is a sandwich valve. Has a flange face on the pipe side, and a flat face on the valve blade side. I bought a new cable and a new set of guts for the valve.
Loosen but don't remove the valve body screws (should be 4). With a little wiggling, you should be able to pull the blade out. Minor clean up, replace the cable, install the new blade, tighten flange screws & give 'er a little test.
Not exactly right, but you get the idea. This valve is a bit different than what I suggested, but it should get you aimed in the right direction.