Consistent unexpected computer shutdowns

Hi all, so the other night I heard my computer computer running on high the moment I went to bed, woke up, and got out of the shower. Upon inspection (where it was still running high) my computer screen wouldn't turn on. Now this has been a problem before, but seems to be associated with LG monitor. So, per usual, I had to pull the power plug wait a few minutes and re-insert it. However, nothing happened for 20+ minutes of mostly waiting. So unable to see an alternative I had to power the computer down. Upon a restart later that day I got the expected message that my computer had to shutdown and go through that alternative loading process. The problem comes in over the last two or three nights where I have gotten on my computer the following morning only to find that it has shutdown again due to an unexpected error and has reset itself. My assumption is a heating problem but I haven't heard it run on high like that since the first night. My computer is a HP e9140f AMD Phenom(tm) II X4 925 Processor 64-bit desktop running on Vista that I got about 4 years ago, if that helps alt all.

Any insight into my problem would be greatly appreciated.