howcan I fix my 2002 vauxhall zafira 1.8 ?

On a cold start the idle is low ( 600 rpm) then
the idle rises to 1500 for a few seconds then
back to a low idle. This happens every 30
seconds or so for the first couple of minutes
of the journey. This stops after the engine is
warmed up, but the idle is still at 600 rpm.