Look down the throttle body and see if there's any oil down there. If there is, a blown plenum gasket is the problem. It lets the intake suck in oil which causes you to lose power, hurts fuel mileage, causes you to lose oil, and it is a common problem on these engines. With over 200K miles on it, I think it's likely the plenum gasket is blown.
This will fix it.
The stock plenum plate is the problem. It's made of steel whereas the intake manifold it is bolted to is aluminum. This means they will expand and contract at different rates when they heat up and cool down. This makes the gasket blow. The Hughes kit has an aluminum plate that fixes the problem. It's not enough to just replace the gasket.