best plug for a 59ci merc

[fs5]

what plugs do you guys run in your 59ci mercs over there?
i currently run ngk bp8h-n-10
my motor is a 55 seapro and they don't have a thermostat fitted and was wondering if you can get a colder plug for these motors.
i find my motor runs better ,when i hold it W.O.T.after a minute or 2 it seems to pick up a few more revs.
could this be plug related ?
thanks

[Mark75H]

You want a hotter plug, not a colder