I have a 2000 mercury marquis with v8 it runs fine for awhile then usually on an incline it looses power and backfires a little , I pull over an Rev it a couple times and it will start to run normal a
Try running a good fuel system cleaner through a couple of tanks of gas. I like Seafoam and Chevron Techron.
Give it a tune up, loss of power and backfiring might mean something's amiss with the spark/ignition, the backfire caused by unburned air/fuel mixture igniting in the cat, you'll have to run some diagnostics, maybe the problem is in the PCM and/or sensors like camshaft sensor). Any trouble codes?