Machine randomly freezing on two different HDDs, is fine on another

Let me explain my issue here.

I work at a small business computer store. I have a machine in right now that does not seem to work properly. I have taken the customer's HDD out and imaged it to another HDD. All of the customer's data stayed with the machine. I sent him home, and the issue continued.

I then installed a new HDD (brand new, not re-certified), installed Windows with the customer's licence, and set up his dial up account and email account. After this the computer seems fine. The customer took the computer for a weekend and said that he had no issues with it.

I have the machine again, to install the original HDD, with his original Windows. I have put that back into the machine, and lo and behold, the machine continues to freeze up when exiting any internet application, as well as adobe photoshop.

I am at a loss with this machine now. I have changed the PSU, the Modem, the DVD drive, the HDD, installed new Windows, and left his data out of the picture. I do not know what else I can do.

Is there anything that anyone has come by regarding an issue like this? Or is this machine destined to continue being a hassle for myself and the customer.

Thanks in advance!
 
I have checked the memory with Memtest over the course of a night, and received no errors at all. I have not actually replaced the ram with new modules yet, as the test came out clean for several hours, until I shut it off.
 
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