hi could anyone suggest to me why when i ride around easily i get no chain slippage but as soon as i get out of the saddle and put a bit of power on the chain slips, the cassette is new sram and the chain is new pc990 and the chainset is new. what i find weird is this has happend to me with two differnt bikes using sram pc990 chains but on a bike using a shimano chain its been fine could anyone tell me if they have had the same experience

No, I've not.

Is the chain the right length?

hi how do i check if the lenght is right

is it slopping around everywhere when you're in the small/small combination ie granny ring and smallest cassette sprocket?

No i copied the lenght of chain from another bike with nine gears but could you tell me what lenght it should be for a kona cindercone 2003

oh dear oh dear.

I seem to remember something about wrap the chain around the biggest ring front and back without the rear mech so there is no slack chain then plus two links and voila! you have the correct chain length.

There could be a whole other reasons why your chain slips. Does it jump or slip? You could have a bent rear hanger, the rear or front mech is not properly set up or your front rings need changing.

I'd put money on your middle ring being knackered on your chainset

I have my chain just long enough that it will run big chainring to big sprocket without ripping off the rear mech. You shouldn't ride big to big as it puts a lot of sideways strain on everything, but it's nice to know it will survive if you go big to big by mistake. any longer than this and there will be too much slack for the rear mech to cope with when riding any where near small to small. Which you shouldn't do for the same reason. So wrap round big to big plus 2/3 links seems about right.
If the drive chain is newish, (less than a few hundred miles) it should not slip. You sure it's not the freewheel about to fail ?