I have a internal coolant leak on my 1999 Ford F-150 v6. Where should I look for leak?

I have already ordered head gasket kit to replace all seals. However, with system 8 below, I was wondering if the leak could be on the intake manifold opposed to the head gasket or some where else I should be looking.

Symptoms are as follows:
1.Fail to fire on cylinder one error code
2. Only cylinder one shows signs of coolant leak.
3. No signs of coolant in oil; however, signs of oil in coolant compartment
4. All spark plugs show signs of over heating, but not bad.
5. Can hear cylinder one failing to fire when idling, with the occasional backfire.
6. Distinct sweet smelling, white cloud exhaust, that lingers in the air.
7. Once engine is to temperature engine runs smoothly even if idling.
8. Occasionally exhaust clears up when I am driving at city speeds and after idling again it stays clear without overheating. I suspect that this is happening when the thermostat opens but not certain of that.
9. Have had the signs of the occasional air bubble in system, where heater stops working and then engine starts to over heat. I turn off truck and open reservoir cap to confirm coolant then close. I start truck up again and every time I do this everything starts working fine.
10. With driving truck about 200-350 miles a week I need to add a gallon of coolant about once a week.

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