Browsers have a mind of their own!

Hello,

hopefully someone can help me...

for or a couple of weeks now my browsers (I use chrome mainly, and Firefox) have been behaving weirdly. They keep going to full screen on their own and when I minimise it pops back full again immediately, also they crash a lot and now and then the mouse pointer goes a bit mental, and when I’m on a couple of websites, when I click to go to a different part of the site, it redirects me briefly to r.srvtrk something or other (it doesn’t stay there long enough for me to copy the address Properly!) then within a second or so goes to the homepage of the site I’m on.

i use Avast and have done a full scan which says there are about 12 files that it couldn’t scan but apart from that apparantly all is ok. Google isn’t helping me much, or more accurately I don’t really know what I’m googling for! Some people say it is adware but avast should find that if I have it?

i am on Yosemite 10.10.4 if that makes any difference?

does anyone have any ideas I can try or know what’s up with my laptop?! :-)