I have a 2004 Nissan Altima which keeps lossing power. It doesn't shut off, but when driving it will loose power and you can get no REV from it. When I turn it off for awhile it is fine. Also the service engine soon light is always on.

. Check for codes--AutoZone will do this for free.
. Check coil--AutoZone will do this for free.
. Change fuel and air filters.
. Ensure spark plugs and plug wires are in good shape.

Look at your Cataltic converter it could be clogged. I had same problem with jeep. I had it tested (removed converted and installed test pipe) and it fixed my problem.