Although other spells can be sacrificed to cast healing or inflict spells spontainously, the spell "Invisability" can't be. This is the only spell I seem to be having a problem with, though my cleric is not high enough to have access to the other spell avalible in the illusion domain, so whether it is just the spell or the domian is unknown.

Domain spells can't be sacrificed for spontaneous casting. It's not a glitch.