"Ping" is back!

[SA-AUII-MAX]

Hi, my name is Max and I’m a Falco(n)holic living in South Africa.
I own a Fairmont AUII 2002 model with nearly 200.000 kilos done. Having a problem with the car, I checked this web-site intensively to get more info regarding pre-ignition or pinging. Realising that there are many leads regarding this problem, none of them is putting my mind at ease. I smashed my catalyst converter not so long ago. As it is rather expensive to replace the cat-thing and the whole exhaust was due for replacement, I decided to leave the cat out, gain some horsepower, and installed a stainless free flow system. The CO (Hego) sensor is placed before the cat so no problems were expected. For a couple of days the Fairmont went really well but started to “ping” driving at light throttle (1500-2300 RPM). Was told that because the cat is gone, the speed of exhaust gasses increases and the temperature of the exhaust gasses decreases - trying to “confuse” the CO sensor. No engine warning light is coming on though.
To compensate I bought a K&N filter. This worked for a couple of days but the engine started pinging again. Installed a “mini-cat”, a 5 cm long piece of tubing installed where normally the CO sensor fits and fitted the sensor at the end of the tubing. Because the restriction from the cat was gone, I took the CO sensor out of the direct flow of exhaust gasses, to give it more time to “read” or “smell”. Helped for a couple of days, then... the “PING” was back! Next up a Unichip was installed, and yes I know some of you guys hate the thing, but I had to control the timing in some way. Got a dyno-tune, more horsepower under the bonnet and better fuel consumption. Great, but again after two-three days, “PING”! (read £**%)
Retarded the timing (6 to 8 degrees), made the fuel richer and tried again and again. Right now the timing is retarded so much that the power is flat, the car is using too much fuel and still “pings” sometimes - I’m starting to believe that it is not really a timing issue.
I use 95 fuel and when the car is under heavy load, towing ± 2.000 Kg, one would expect it to be worse but there is no difference in the problem. Checked the car with a proper code reader at the Ford garage and got a code 0325. Checked and cleaned connections to the Knock Sensor, tested the sensor itself and did the voltage test. All fine but “PING” is still remaining and I’m running out of options.

Is there somebody out there that had similar problems and got it fixed besides buying another car?