Pressure in the shower drops on turning diverter

I recently started having a problem with my shower in the main bath upstairs. Whenever I turn the water on, no problems. When I turned the diverter to shower, the water starting coming out fine and then the pipes would make a wierd sound and then the water would pretty much shut off (a small trickle out of the shower. Happenes when shower head off too!!!). When I turn the diverter back to tub, the water comes out of the faucet fine. I can get the water to come out of the shower ONLY if I slowly and barely turn the diverter on so that there is water coming out of both the shower and the faucet. If I turn the diverter far enough to almost cut the faucet water off, I hear the noise and the water flow stops. Note: This also happens with the shower head off!!!

This is a Gerber shower valve with the hot cold and diverter all in a row.

I opened the Diverter (11B-4D Gerber Stem). Following URL shows the drawing.

http://www.************.com/invt/7144116

I replaced all three stems last year. It worked fine till last month. I opened the diverter. Nothing suspicious except one rubber washer (not one at the end of the stem) dose not stay in it's position when stem is in open condition.

Pl. let me know how can I fix it.
Regards

SOunds like that washer is blocking flow. It needs to be attached to the stem. Is the screw loose and floating around? It could be part of the problem, then fall back when the flow reduces. Full flow can push it to where it will block things.

How to attach gasket to stem

Jim, Thanks for the reply. This gasket is suppose to be press fit and not expected to come out?

How can I attach gasket to stem. using Glue?

Or getting new Gasket which can be pressfit?

Regards,

I don't see the internals of many valves...the ones I've played with use a screw to hold the washer in place. If the washer is supposed to be held in by friction, you may find that only an OEM part will work. See what the pros say with their much greater experience.