Mac Pro 5.1 random shutdowns and boot up trouble

Hi All

I have a Mac Pro that has been causing some issues recently that i fear are getting worse.

It started off with intermittent shutdowns then it would re boot, eventually it appears to have escalated to at times not booting up at all or stalling upon booting up. After about an hour of persistence and trying different things it would start but as i need it for work this is no longer satisfactory so turning to this forum for some generous help.

I have tried replacing the lithium battery, resetting PR Ram and SCM booting from different drives and OSX versions. Replacing RAM, None of these make any difference.

I did notice not long before i had similar issues that appeared to be fixed when loading an app that allowed me to increase the fan speed, as it did get hot here and noticed it did seem to contribute to a similar issue.

Another possible hint may be that when connecting some USB devices to the tower (mainly the Cinema Display USB that ran other USB devices) that the display would be blank.

It seams as I fix one issue another appears .

I do have another Mac Pro of the previous year that displays none of these issues using the exact save peripherals so I an certain it is confined to the Mac Tower itself and no peripherals or drives.

Thanks in advance for any contribution.