I've had the same problem with my 620m. it eventually even had problems swiching on at all even using the brick on top solution.
The main problem was the on/off switch itself. I disassembled the karcher and removed the swich (if your doing this take a picture or two of the wiring before you pull the spade connectors off & make sure ur not pluged into the mains when you do this. the pics are always good reference when putting back together to ensure you reconnect correctly i make this common practice for any dismantling job i do).
once you have the swich out you can further disassemble this to access the actual internal workings of the switch by removing the black cover (careful not to break the lugs off, do one at a time in rotating order & for gods sake watchout for the springs when you take the cover off). My guess is (as was with mine)there has been previous arcing at the live or neu terminal which has melted the plastic latch mechanisim, hence will not stay latch on, latch off as it should.
You have a couple of options 1. Buy a new switch 20-30 quid or 2. repair it (go on give it a bash nothing to loose!)
If you are going to repair it, it may depend on how bad the damage is. Carefully remove / trim the melted disformed plastic only (it will have probs melted round the copper terminal )with a stanley and scrape the arced terminals to allow a better connection when operational. reassemble everything back together and the switch should now sound like the latching is clicking in & out of place (this may however not be fool proof. mine was badly damaged and seemed to latch on/off ok but once reconnected to the karcher it didn't repair the latch problem but at least gave me power again using the brick on top solution. I think the brick on top solution had been used for so long my swich had prob more damage than yours. however you could be in luck.
I was now happy my 620m would work again all beit i'd need a brick handy, but i had confirmed the problem was the on/off swich itself. once i'd confirmed that i decided to just go the 20 quid and i've just ordered a new switch to fully fix my 620m. like i said mine was badly damaged you might be lucky. if i were you id give it a go before ordering one.
if you are looking for a switch for it i found one for it on www.cleanstuff.co.uk they seemed the best price think it was 17 quid odd in vat. (make sure it's part 8)
I'll try and let you know how i got on after fitting the new one.
Hope this helps...let me know how you got on!
Stu
Stu Barr, April 2009