5.2 intermitent rough idle.
 

Just bought my 99 5.2 4x4 is high mileage at 285000km (177k miles). Runs with a knock under load, but starts and works well. When the truck is started (cold or warm, doesn't matter) it idles nicely then after a minute or two it starts to stumble and goes really rough for a minute or two and smoothes itself out and runs without issue untill I turn it off. Restart after 10 minutes and problem repeats. Oil is new, no smoking issues, plugs are new, (old ones weren't too bad), new distributor cap and rotor, plug wires appear newer, fuel in tank is good.
I'm thinking it might be a fuel rich condition (why?) or ignition retarding (why?), any ideas?
Thanks,
Gman