Same problem. It seems to involve the button itself. Sometimes I can move my finger to a slightly different area of the button and it stays on a little longer. Very frustrating. I love my Dyson 35 but want to through it out the window when it acts up.
The filter needs cleaning. Try running it without the filter in. If it works fine, the filter needs cleaning, even though it looks clean. This happened to me.
Bought the very first hand held Dyson had the same problem was told there was a fault with the on of button as out of warranty they gave me a small discount to purchase the new model as the problem had been rectified bought it ,out of warranty same problem never a hand held Dyson again