The problem seems to be that the cold water pressure is greater than the hot water pressure. So when the shower head is set to dribble, the cold water starts to push the hot back into the supply pipe and just cold water comes out the shower head.
The fix is to use a check valve in the PEX as close to the faucet as possible. This check valve stops the cold water from pushing the hot water back.
Here is some added reading on the subject: http://www.rv.net/cforum/index.cfm/f...g/24177321.cfm
The check valve in the added reading is not available from the supplier listed, but, you can get a SHARKBITE
You just cut a small section of the PEX out and push the PEX into the SHARKBITE. Sharkbite check valves cost around $12.00.