|Remove the ball, open up the cover for the mouse from the bottom. If the screw holes aren't obvious they are under a label - apply some pressure to the label with your finger to find the places the label has nothing under it and moves inward - that's where the screws are. There are usually tabs that hold one end of the mouse together once you get the screws out and you may need to shift the halves away from each other there.|
Check for mung or fuzz at the ends of the roller bars, and beside the third spring loaded roller, make sure they move easily. Check for mung or fuzz at the wheel end of the roller bars in the housing for the wheels. Check that the switches are free of mung and move reliably. Isopropyl or methyl alcohol helps get the stickier stuff off, and may get the switches working better.
If that doesn't help, if this is a corded mouse, it is common for wires inside the cord to break after the mouse has been used a lot and/or if the cord has been pulled on, usually right where it enters the mouse. At first the wires are broken but the insulation is still intact on the individual wires, so the wire ends still touch each other - you get intermittant problems - sometimes the mouse works fine, sometimes it doesn't, depending on the position or the cord. If that is the case, wiggling the cord where it enters the mouse will get it to intermittantly do strange things. You can open up the cable sheath and repair the affected wires by soldering or splicing, and/or pull the affected section of the cord inside the mouse further, but it's a fiddely job.