I have to agree with this. I've been repairing computers since the Apple II.
Sadly, many electronics repairers have become glorified board swappers, following a flow-chart to swap modules until the reproducible problem goes away.
The board swapping principle started in the PC repair industry and has now transitioned into the TV repair industry too. Whilst swapping modules with 'guaranteed' new modules has it's merits, the downside is that repair techs become dumb drones, only able to follow documented instructions and unable to look at a fault logically and apply troubleshooting to the problem. Under these circumstances, I'd be asking the customer to show me how it happens, and observe what is going on. I'm tending to believe that they are not doing this and instead simply take the machine into the back room and run continuous diagnostics on it and then give it back with "no fault found".
To the OP: I'm wondering if you have tried a plain cheap non-Apple mouse. Apple's last two mouse designs use touch sensors that may be registering scrolling gestures.
Of course we still need to establish what machine we are referring to. I took the post to mean a MacPro "tower" machine, and then after experiencing this problem, he's bought a laptop.