Engine misfire stumps mechanics

1998 Mercury Mountaineer, 78K miles, 5-litre V-8, started misfiring several months ago but did not trip check engine or throw error code. Problem varies from mild shaking/rough idle to severe 'bucking' backfire, etc. Gets worse pulling grade/under load. First garage gave me new plug wires, fuel filter, 1 coil pack, checked compression - Ok, all plugs checked-ok, valve springs checked-ok, timing checked-ok. I've tried several rounds of injector cleaner through the tank. Seemed to help for little while. Second garage put cleaner directly in injectors and checked fuel pressure-ok. Also put in new fuel pump (sounded like it may be going out). No signs of spark problems. Fuel consumption up dramatically (less than 10 mpg). Any help greatly appreciated. Thanks.