I would be inclined to believe the issue is with your heatsink mounting, sounds as though it has become loose. I'll suggest checking that the heatsink is firmly attached at all four corners to the Motherboard/CPU.
I have checked all 4 scruws and they are OK fastened. Still I have the problem. Sometimes when i try to start the machine it won't power on until i give it a little kick with my hand. Then it starts and run for a while. This never happends when it is lying down.
Your analysis was correct. I dismounted the heatsink, put some new paste on tne processor and remounted the heatsink. Now the machine has been working perfect in upright possition for more than a week.