Typically when it idles ok and backfires under load, it's a sign that one cylinders has poor spark. What resistance reading were you getting from the coil packs? It should be between around 0.15ohm and 2.15ohm. Otherwise it is possible you have a weak or bad spark plug.
I would recommend testing the coil packs and spark plugs, then make sure when reinstalling they are seated properly. Otherwise the issue may be further down the coil wiring.