603 missing at start up.

Lately when starting up in the morning the engine fires only on 5 cylinders for about 10 seconds before all 6 are firing.

Is it possible that if a GP is not heating, the cylinder it serves will not fire for the approximate 10 seconds I mentioned above, any ideas?

Yes, the block heater is plugged in for 2 or 3 hours before starting, but I think lately the block heater has deminished ability and probably need new and doubt this has anything to do with missing one cylinder! BB