I have had numerous re-occurences of finding WIN32Tr\.\erHijack following scanning and removal.

Can anyone provide any insight on why it may be coming back or is not being removed once and for all?

Some malware can only be deleted upon reboot, so if you normally hibernate, you can try full shutdown/power off and see if it gets deleted with the reboot. If it persists, read the instructions in my signature about posting a log in the HijackThis forum. Copy/paste or link to this thread to describe the problem, and somebody there can help you with malware removal.