Mystery, My slide bar moves in tandem with my cursor

Morning folks, Peter in UK here.

Over the past several months if I move my cursor over to the slide bar, the slide bar then follows my cursor movements elsewhere on the screen. So for example. Go the Right to scroll down quickly, scroll down, move off the slide bar and everywhere up and down my cursor goes the slide bar moves the screen an equal amount.

Quite maddening actually :-)

I have an iMac running Yosemite 10.10.5. I have researched this quite a bit and seen that some have this fault in adobe (which I believe I have on my computer but whenever I download it, it soon disappears from the computer??)

I have a very low level user of the iMac. I search the web, use eBay and send emails. thats about it. so no high level programs on the system.

Any help much appreciated.

Thanks, Peter