missing between 1000 to2000 rpm
 

my subaru2002 sti jap spec hesitate when passing through 1000 to 2000 rpm i diagnose it with delta dash scanner but cant find any fault can any body advice me what wouldf be the possible cause

how old are the plugs?

they are 1month old only
this misfire start after 1minute of start
if i stop the engine and start again then there is no missing but after 1 minute it again start to mising
and also i log it with deltadash and i find that when fuel pump duty cycle drops from 100 percent to 33 percent then this problem starts
please help me