It could still be the head gasket My old MG ZR 160 had a blowen head gasket but no one could find a problem with it but it had the exact same symtoms as your describing and what it was as the car warmed up it let off bubbles in to the water then after a long run it would be ok till the next time you started the car . Or you could just check your pressure cap it might just be that gone faulty. Hope this helps