Well we are lacking alot of information here to really narrow this down, like hardware, settings, OS version, did you update drivers, what is the screensaver, have you gone into the screen saver's panel and set it to a different resolution, does this happen with all screen savers, etc. etc. etc. There is ALOT more to the puzzle then just your quick question, sorry.
So to eliminate what causes 95% of all issues usually that are 'underlying' things I would do the following;
(This is my monthly "to do" list when I service computers and what I recommend)
Did you install all Windows Updates? Including OPTIONAL except BING? Check them and repeat till ALL are installed.
Download and run Slim Drivers (http://www.driverupdate.net/downloads/SlimDrivers-setup.exe), or try Driver Booster (http://www.iobit.com/en/recommend/sd.php?name=driver_booster_free) install all the latest updates but you don't need to reboot until you do the last update
Go to www.filehippo.com, download Malwarebytes, go to CUSTOM Scan and set it to do a full system scan (by DEFAULT IT DOESN"T DO THAT, and AntiVirus doesn't pick up alot of malware) - this resolved almost ALL other similiar posts to date as most had Malware the AV didn't pick up.
(( NOTE: Many 'FREE' screensavers come with alot of malware / adware / annoying packaged stuff included, some do things like "changing my resolution" to make you think your computer is 'broken' then 'buy' their pop up 'warning' which is fake, the malware itself is causing the issue. ))
Remove whatever AV your using and download AVIRA, AVG, Comodo or Panda go to CUSTOM Scan and set it to do a full system scan (by DEFAULT IT DOESN"T DO THAT) - this repeatedly has resolved alot of people issue relying on MS Essentials.
Repeat the AV/Malware scans till the system comes up clean.
Download and run SPECCY, highlight, then copy, and paste here the first tab to show your idle temps
Download AUTORUNS and turn off anything NOT critical on startup, so you don't need to run JAVAScheduler to check for Java updates everytime you start your computer for example. BE CAREFUL AND KNOW WHAT YOUR TURNING OFF BEFORE YOU DO IT.
Open Computer, Can Air dust out the bunnies and use paint brush on the vents, coolers, fans, etc.
Go through and provide a response on each point, don't SKIP IT because "I ALREADY DID IT" , just try doing it again as of THIS post so we know NOW what is happening as compared to BEFORE you posted.