Pinging NG?

[duandcc_forums at cox.net]

The car is a 1987 Coupe GT Special Build (2.3 NG engine). Ever since i bought it it has has a pinging problem (even with 93 gas). Here's what I've done so far to try to fix it:

1. Multiple tanks of gas with Techron addative.
2. Replaced spark plugs (first with Bosch Plats, then with NKG non-platinum triple electrode).
3. Replaced knock sensor.
4. Replaced air filter (as scheduled)
5. Looked for/repaired a  vacuum leaks (bad seal on oil cap).

I've had enough guessing on my own and am almost ready to take it to the DEALER. The car idles perfectly with no signs of vacuum leaks (not to say that there aren't any, just that it idles fine). The distributor still has the factory locking tab on it that locks it at 12*, so it doesn't look like the PO messed with the timing. I have considered changing the cap, rotor and wires, but I'm not sure this would be of any use, would it? Also, would you suggest trying a set of plugs that is 1 heat range cooler? Or is that the wrong way to "fix" the problem? Any thoughts or suggestions would be helpful.

TIA,

Dave
1987 Coupe GT Special Build