Monitor signal not detected and boot restart

Hello, I was just wondering (again) whether anyone would have an answer to a specific query I am having with my computer. Previously I have had some very troubling errors with such, which can be found at on a previous post I have made. Upon receiving advice from someone upon this website to acquire a new hard drive and have such installed (which I have done) my problems seem to have changed somewhat. Currently I am experiencing randomly timed monitor non-detection of the signal input, the VGA cable is perfly connected and the is then rendered in sporadic modes of being able to be turned off immediately by pressing the power button, to taking the normal several seconds or so (which I researched could be part of a windows failure), though this can change with a further problem I will explain. This non-detection is often expedited with increased usage of different applications, as I had a game downloading and then activated a new tab upon google and it close to immediately crashed, which lead me to believe it having a GPU problems, which I have previously experienced if further looking into my previous post is done. Continuing on from this, I have a random problem; with the monitor non-detection of the signal once again, but upon these events, the computer will detect the signal and reboots at an average of three times and say that windows could not restart correctly. This problem is currently making my PC very trying to use as I can only spend at maximum 1 hour upon the machine without these problems coming up.

Any assistance or insight provided upon this subject would be greatly appreciated.

Thank you.