Fred and LML,
I have seen this problem several times before and found the cause to be the emergency shuttle valve ball going into the emergency position for some reason. The reason can vary from plane to plane depending on the schematic of that specific system. For example, if you are using the emergency blow down accumulator in your system then the valve that is used to blow the gear down is probably leaking a tiny bit internally. There can be lots of reasons but the little ball in the shuttle valve is probably what is doing it.