Detonation problems: Help?

[95GTS]

For some reason I'm getting detonation in my 95 5.0. I've had the car maybe a month. It's only got a little over 78,000 miles on it. When I bought it it was bone stock except for FMS 9mm wires. It was detonating then so I since have done a tune up and added a few things: New K&N cone filter, new spark plugs Motorcraft 42C gapped at .054, new cap (brass terminals) and rotor, and new Magnaflow mufflers. Still getting detonation so we checked the timing and it's at the stock 10* (and yes that's with the spout connector pulled out). Also, this is all on 89 octane fuel and I've run 2 different types of fuel system cleaner thru it. So now what? What could be causing my detonation? The only thing we can think of right now is maybe the stock cats are, for some strange reason, getting a little backed up and causing too much back pressure which, in turn, is causing the detonation. I wouldn't think the cats would be going bad already (stock H-pipe). I plan on getting an off-road X-pipe anyways but I would like to figure out what the heck is going on with the detonation first. Any ideas?