Good old Xbox classic,
I have one also, soft modded it and use it as a media player now...
Anyway with regards to your issue..
The micro switches could also be faulty or even the motherboard as this is where they connect to..
I would recommend you check out some of the Xbox Modder communities and talk with people there.
More then likely you may also be able to source parts or get advice on how to fix this issue..
Here are a couple I have used in the past for my modding..
http://www.xboxscene.com/http://www.xbox-hq.com/Maybe search there or ask around the forums, I am sure you should be able to get some assistance.
Best of luck!