Computer lagging/skipping now while playing games.?

I've had issues like this on a pretty powerful computer (i7 4770K, 32GB RAM, 4-disk raid-0 which is almost as fast as SSD, r9 290), turned out to be a bad update for my virus scanner that turned my pc into a sluggish thing... reinstalling the virus scanner had no effect but replacing it with another one did.

But really, it could have a couple dozen reasons. First of all, I'd to a thorough cleanup of your computer. Check for viruses and malware (don't use pc tune up software as they can seriously mess up and slow down your computer even more, NO reg cleaners unless you're an IT engineer and know exactly what everything in the registry is used for). Make sure you have enough space on your hard drive so that your files don't become ridiculously fragmented (cleaning out your cache and temp files is good practise but won't speed up your games).

Do a system restore and see if lag is gone... you might have picked up a bad windows update or bad driver (latest drivers are good practise, but can introduce bugs and performance issues).

Check if all your hardware is still in good working condition (do a stress / burnin test to make sure and check your hard drives smart info for errors or warnings), check your memory and keep an eye on your performance logs to see if nothing is using 100% of your memory and/or CPU.

Check the temperatures of your chips (cpu, gpu and others), there are free tools you can download to keep an eye on them. If they get too hot, internal protection systems will throttle them down. You may need to clean your fans and other computer internals and/or replace cooling paste between the chips and fans if it's over 2 years old. Add some case fans or a cooling pad for laptops if you notice things are getting hot and make sure no air intake and outlet is blocked.

reduce your startup programs in windows (start -> run -> msconfig) to free up resources and make your computer generally start faster.

... doing all this should solve your problem or at least give you an indication where your problem comes from.

How long have you had it? Dust can be a problem. Also try bringing up task manager or download MSI Afterburner. Either one can monitor your CPU, RAM, and MSI can monitor GPU usage. If one is maxed out, that's the issue.