Laptop turns on and off in 5 seconds then on again by itself

[lymbridge]

toshiba Laptop turns on and off in 5 seconds then on again by itself why!
when you turn it on it gets as far as the toshiba screen then turns off and turns itself back on and does the same,time after time after time ,till power is disconnected !
this is on mains and battery !
why

 

[BILL_S]

Well, not quite the same, I have a Dell out of three Latitudes I bought that WORKS perfectly, BUT When you press power, it is blank but making noise for 40 seconds or so then boots up normal saying it's in servicing Mode...press Fn-X to cancel.
I can't tell you how many ways I tried to press that sequence and get something to happen! It still does the same thing even if I hibernate the drive! It runs like normal after it posts and boots!

 

[Tmagic650]

We are still suspecting a bad system board unfortunately