Cold start misfire

[Bushbasher]

OK, this has been bugging me for a few weeks now and I'm still not sure what the problem is but I'm guessing it HAS to be head gasket....... anyway.... on cold starts the motor is only running on 5 cylinders as #3 isn't firing. Problem is, I can't figure out the problem. It's not a dud plug as they've been changed twice, it's not the coil pack as it's getting good spark and it's not the injector as it's doing it on both gas and petrol alike. So that just leaves head gasket leaking coolant into the cylinder or maybe a sticky valve right? Thing is, it doesn't appear to be losing any noticable abount of coolant and the cooling system isn't being pressurised. Off idle at about 1500rpm it seems to come good but dropping back to idle #3 drops out again till it's warm. Working on the theory that it's coolant in the pot I revved it and held it at 2500rpm for 10-15 secs thinking that would blow the water out and it should run clean but it still dropped #3 again after I tried this. Also, if it's a sticky valve surely there'd be some noise to indicate something's not right but it runs quietly as normal so all in all it's got me stumped. Anyone got any thoughts on my little dilemma as to what to look at next or my next coarse of action? Also, if it IS a very small coolant leak is there anything I can safely put in the system that'll seal the leak up without causing any other detrimental side affects so I don't have to shell out for a head gasket replacement?

Cheers
Bushbasher