I've had mice that do that. It is the switch. This is called a "micro-switch". Here's a replacement:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=370198562470

This particular one comes with a bar across the top. This would need to be pulled off. These same switches were used in Apple UniDisk 3.5" disk drives.

Click to expand...

Dennis is that the correct switch for an ADB mouse? I ask because the original Macintosh Mouse used a larger microswitch than the ADB mice. The ADB mice used the same switch as most (all?) of the PC mice, so that's probably the correct, later switch, but it's always good to check the dimensions before ordering.