This doesn't sound right at all. It's behaving like a classic valve, not a retro valve.
Have you cleaned and oiled your valve and level 10 lately? It sounds to me like you might have some dirt issues.
Another thought...try tightening the field strip screw a little more. If that doesn't help, then try it only hand tight. I've heard of some retros having issues if the valve is not lined up just right.