I have an HP elitebook 8530p crap, and I had the same problem
(the X server thought that one button was pressed, but not released),
and it has turned out that an 'rmmod psmouse' "solves" the problem.
So I backlisted the psmouse module:
#options psmouse proto=imps
I suppose that you can play with the "options psmouse proto=imps", but it doesn't work for me.
It's easier to buy an usb mice, and forget the touchpad.
So I think this bug is related to some kernel bug/regression somewhere in the psmouse/synaptic code.
Or buggy hardware.
(The problem cames out _maybe_ often, if the laptop's temperature is higher, but it's just a feeling.)