Hi. At first, reading your description, I was thinking that maybe one possibility is bad wires (i.e., spark plug wires) or distributor cap, since only the back two cylinders seem to be affected -- anything else (except also the spark plugs, which you changed) would be common to all four cylinders. Then, in your p.s., you mentioned that this was since you put in new "Magnecore" wires ... ah, ha!
Any chance that either the wires are defective, or your installation created a problem, e.g., you didn't snap the terminals in (either the distributor cap, or plugs) properly, or you've got a wire that's too close to a ground and it's bleeding off voltage?
Example: I once routed a wire too close to a piece of the engine, and over time it began shorting out - the engine was running really rough, and it stymied me until I picked up the wire and, all of a sudden, the engine smoothed out. The wire was actually shooting "sparks" through its insulation into the engine's metal.

Also, just FYI, higher seasonal temperature exacerbates an engine's likelihood to ping -- very often, a car that pings in summer is OK in the winter.

Have fun.