Missing, loss of power, hard to get it past fifty mph

Went over a pass the other day and had to have the vehicle in third (pretty big pass between Idaho and Jackson Wyoming. Got gas at Sinclair station (ethanol free?) And returned home to Idaho. Next day engine was misfiring and did not want to go over fifty on a flat. After five miles pulled over checked plugs and wires. Still bad backfiring. Bought new plugs (double platinum) installed, added Seafoam and again filled the tank. Drove around awhile (half tank) let it sit a day. Three days later same problems. Valve four has no engine change when wire is removed, but all coils are sparking. Help, I cannot afford a new beater.