pitch problems

My moog eitherwave (not kit built) has been acting up. It works fine when I'm in nomal playing distance but when I move away to play keyboard or guitar it senses my movement and the pitch lowers and gets higher. It's driving me crazy as I had worked fine when I first bought it. Do I need to open it up and make adjustments? What to do? Any suggestions appreciated. Also I have taken it off the mic stand and set in on a table which seemed to solve the problem but I need to use a mic stand for jamming and gigging.

As far as I know, that's normal behaviour. When you're away from the theremin, it may be silent or it may detect you and make sounds. I think it depends on the room layout and the layout of objects close to the theremin (tables, chairs, walls, etc.), more technical people will be able to give you more details.

If you don't want it to do that, the easiest thing to do is place the audio cable over the volume loop to mute the theremin when you're going to play another instrument. 

AlKhwarizmi is right. That's normal behavior.

More expensive theremins like the Burns B3 Pro, the Moog Etherwave Pro and the tVox tour have a mute switch. But muting it with the audio cable over the volume loop is the cheapest solution.