1995 Pontiac Grand Prix Misfiring & Now Won't Start

1995 Grand Prix 3.1LV-6 was stored for 6 months with 1/2 tank of gas & ran great before. I started driving it again but it ran rough on start up, bucking a bit going up hills. I assumed water in the tank & used Berryman's B-12 thru 3 tanks of gas-no change. It was worse the faster I went - at 50mph or less, it ran a bit better.
One day driving to work it suddenly shuddered, ran very rough at all speeds & idle. It also had a throaty, rough exhaust sound (popping occasionally) that it did not have before. This happened just all of suddened, no warning.
I changed the plugs & plug wires & tested the ignition module at O'Reillys (tested OK) but problem persisted. I left O'Reilly's & it still ran rough but started shifting gears very hard. Suddenly it bucked hard once or twice & died. I had gauge lights & it cranked, but no restart. Not sure if coil packs got back in right order on the ign. Module. Plug wires are on correct coils electrodes. Car still won't start.
I then replaced the fuel filter and all 3 coil packs. Now when trying to start, it acts like it's out of gas. I put 4 gallons in, so I know it's not. It will crank & turn over, hesitate, & I hear an occasional muted pop from the tailpipe. It intermittently tries to catch and fire up, but never does. I can hear the fuel pump hum for a few seconds when I first put the key in and turn ignition to 'accessory' or 'on'. Any ideas? Help!