Staffy,
Good advice from all here!
Will put my $.02 in, if this happens only when you first start the bike when it's cold (choke on), try this, as soon as you start it, turn the choke off just for a few seconds (3-5 seconds), then pull the choke back on & see if the RPMs stay low, keep the choke on until it is warmed up.
DoldGuy