The exhaust fan on your computer is either not spinning or the sensor is faulty and is not sending feedback to the system management controller. This could indicate a power supply issue if the fan is not being properly powered, especially since at times it would not power on properly.
My overall guess here would be that it's a power supply issue, and you would benefit from having it replaced with a new one.
my 2-1/2 yr old macpro has had problems not starting at all when pushing the start button. for a while the smc start worked, then nothing did. took it into the genius today, started right up and tested perfect using their power cord, wouldn't start with my power cord. brought one of their power cords home as a temp and started up fine so far. so if this fix remains to work, might just be the power cord.
Thanks for comments ... It certainly looks like the Exhaust fan is not spinning - or like you say the sensor does not detect it... Is there somewhere within Mac OS which says if the fan is working properly? - and if it is not working is replacing the power supply unit likely to get the fan working - i'm thinking maybe it is just the fan or the fan and power supply unit that i need... It is booked in for appointment today - which hopefuully will get to the bottom of it - took about an hour to get started today - not good!!
You have taken the next step i was going to recommend. The guys at the genius bar often have spare fans they can substitute and spare Macs they can use to power your fan to do the troubleshooting. This can determine whether the fan itself is a problem.
Conventional Wisdom for Power problems has you removing everything that could potentially be dragging down the power, and building back up until the last component added causes a problem. (It may be that your fan was failing in a way that caused power problems.) Do not be alarmed if they want to massively dis-assemble your Mac.
I think they try not to do large-scale dis-assembly at the counter, but taking it in the back to do it takes them away from the counter where they may be multi-tasking -- helping two customers at once.
Figuring out whether your Power Supply is working properly is a much more subtle problem. But there are some LEDs inside the cabinet that generally light up at power-on, then extinguish when no fault is found.
The power supply has now been replaced by the mac store and all seems to be working well including fan that was apparently not connected properly. However with a new power supply all seems to be working ok... The machine also sleeps and starts up properly which had been experiencing intermittant problems.
All seems ok now !