Help!!! Loss of power!!!

I have a 99 VW Jetta. Within the past 2 weeks it has been acting up. I'll be driving and all of a sudden there's a loss of power...I mean a huge loss of power. I can barely get up a hill. And when it's idling it doesn't stall but it wants to, and the whole car shakes when idling. After it sits for a few hours and I start it up again it's good to go. You never know when it's going to happen again, but it does. My husband has changed the fuel filter, spark plugs, and spark plug wires. I just filled up the tank and put a bottle of fuel injector cleaner in it as well. Any thoughts to what could be causing this, and possible solutions!?!?!?

Investigate and see if there is a check engine light. which there probably is. Get the codes read at an auto parts store. Could be alot of things. Report back with the codes.