1999 Subaru Forester Loss of Power Upon Starting Vehicle

I have a 1999 Subaru Forester that has started having a recurring problem. The few minutes after I start the car, the engine has no power. The car doesn't stumble or stall, but it feels like the gas pedal doesn't do very much. You can still accelerate, but it's sluggish to move. Giving it more gas doesn't alleviate the problem. It also has a check engine light indicating that the catalytic converter is dying, but the light was on a few months before this problem started.