1998 Buick Riviera MISSING, INTAKE BACKFIRES

Engine problem
1998 Buick Riviera 6 cyl Front Wheel Drive Automatic 248K miles

Backfires at low rpm load condition sometimes violently. If rpm is increased to the point of a downshift, the engines runs great. The problem is intermitten but always happens during the condition above. Have replaced plugs, wires, coils, and TP sensor. The problem usually occurs after shifting into high gear, but will backfire in lower gears with same condition.