My pc starts restarting itself whenever I play games of high-end graphics

If the system restarts on it's own it then it's usually software related. Bad driver, bad windows update, missing file.
Windows may be recording the error in the Event Viewer, under System or Application logs. If you know the last time your computer restarted on you, you could go back through the event viewer (if you're not sure how, google it) and see if it reports any Errors or Critical issues in the System log or even Application log. It is also possible Windows is creating .dmp files, if it is, you can find them on your system at this location: %systemroot%\Minidump (for me, windows is installed on my C drive so I find any .dmp files it may create on my C:\Windows\Minidump)
If no minidump folder is there, it is possible your windows isn't setup to create them. You can fix that with these steps:
1) Go to Start and type in sysdm.cpl and press Enter
2) Click on the Advanced tab
3) Click on the Startup and Recovery Settings button
4) Ensure that Automatically restart is unchecked
5) Under the Write Debugging Information header select Small memory dump (256 kB) in the dropdown box
6) Ensure that the Small Dump Directory is listed as %systemroot%\Minidump << where your .dmp files can be found later

A faulty PSU or bad power could cause the issue - power cuts off for a split second and the system goes to turn off, but the power comes right back on and the computer ends up restarting, though, this isn't very common. This is something you may see happen if there power in your house flickers (where all the lights dim because of a bad storm), but the power doesn't go off.

Heat related issues or bad power supply should result in your computer powering off. You'd have to physically push the power button to get it turn back on.

My suggestion would be to do a clean install of your video drivers. I'd suggest making use of DDU, you can download it from here: http://www.wagnardsoft.com/forums/viewtopic.php?f=5&t=736

1) download the latest driver for your GPU
2) download and use DDU. Have it uninstall your current driver in Safe Mode and restart your computer
3) when your system restarts, install the driver for your GPU
4) test things out and see if the issue persists.

If the problem continues, check Event Viewer again, see if anything shows from the most recent restart or is possibly in the minidump folder. If something shows up, you can't just read the .dmp file. You can use BlueScreenViewer, you can find the download here: http://www.nirsoft.net/utils/blue_screen_view.html

Next, hopefully you still have the GTX 750. If you do, I'd suggest to re-install the GPU and see how things perform. Seeing as the R7 370 was the only thing you listed in your post that changed, putting everything back to how it originally was in a working status, this will let you know where the issue may be. If the problem doesn't happen with the GTX 750, then the issue is the R7 370 or possibly the PSU. It would be hard to be certain where the problem lies, because the R7 370 pulls more power than the GTX 750....so the GPU is faulty OR the PSU is going bad and can't power the system when the 370 is installed.