Not sure I like the heat idea - what is it about being in combat for a while that suddenly makes you able to use source?
What if source powers just take way more AP because you have to spend some time concentrating/preparing? Right now, there's a 6 AP max; you could increase the max to 8, but the 2 additional AP can only be used for source skills/spells. Then make source skills take 8 AP. If you use any non-source skill while AP 7/8 are charged, you lose those charges. So, you could skip your first turn entirely and then use a source skill on your second turn (assuming you are still around) or you can forgo the source skill and use regular skills to try to interrupt/kill an enemy who is preparing a source skill.
You might also want to make it so that source skills can't be used outside of combat to avoid people initiating combat with chain lightning, but that could go either way.
