If we are talking about ability boosts and regeneration (and stuff like leaping, increased saves, etc) make it an Alternate Form with the Side Effect Flaw.

If you want to make it a limited duration, apply Fades (-1) and Total Fade (+1) for a net cost of +0 (seeing as you end up with a loss of power, you might see if your GM will float you some Drawback Points to get the Slow Fade power Feat to extend your power's duration).

Otherwise, Flaw all your powers and stat boosts with Sustained Duration (-1 to cost for stats, saves, and regen) which will make your AF a Sustained Duration power. You get your bell rung and lose your concentration; the power boost turns off. If you activate it again, you get hit with the Side Effect again.