Computer crashes always - or never (in safe mode)

I hope that someone with intimate knowledge of the workings of the PC and Windows 7 would be willing to comment on this:

I use a Lenovo (ThinkCentre M55 8810-BV6) desktop with Win 7 in 'safe with networking' mode, with enabled sound and full resolution; regardless of how heavily the CPU, etc. is charged, it runs fine - I can watch several video streams simultaneously, communicate on social networks, download torrents, etc. - never a single(!) crash.

However, if I try to run the machine in 'normal' mode, it crashes in less than 15 minutes, even if left idling, and with most services disabled or set to 'manual'!? And - there's no blue screen, minidump or record in events logs!?

Any ideas about what might be the explanation for this behaviour!?