For about as long as I've owned my Dakota (a 97 5.2 with just over 100k miles) I've been getting P0300, followed by misfire codes for every cylinder. After erasing them, the codes usually reappear within a week or so, starting as pending codes then eventually confirmed and lighting up the CEL. On the recomendations of various people and from my own research, I've replaced the IAC, TPS, distributor, spark plugs (OEM champions), wires, distributor cap and rotor, ignition coil, battery, upstream O2 sensor, throttle body thoroughly cleaned, and probably other things I've forgotten. I've had it tested for vacuum leaks and while some minor ones were found, they were fixed and it made no difference. I also redid the plenum with the Hughes plenum kit. The only symptoms the truck has of misfiring is a slightly rough idle, slightly more so in park for some reason. The rpms in park also seems to be about 20 higher than in drive and fluctuate slightly. Other than that, the truck runs perfectly. No bucking, no stalling, accelerates great, doesn't seem to have any loss of power. I've watched the CEL come on as soon as I come to a stop and hit idle. I've exhausted just about every idea I have trying to fix this thing, I'd appreciate any help.

 

Check the timing of the distributor to the engine. I was getting codes on the odd number bank of my 3.9 and the shop I use had removed the distributor to change the intake manifold gasket and bolts, and put it back slightly off from where it should have been. They corrected it for me and I've had no issues since then.

I do not know anything about your V8, but my V6 has a timing chain that they thought might also have stretched a bit. I'm holding my breath on that one....

Good luck and report back when you sort it out.