Platinum has better spark potential but it wears out quicker. Silver is the best because it is somewhere in the middle and has the highest conductivity of any metal, even more than copper. The only reason they don't use silver is because it is way more expensive than copper in auto applications. It's also not readily available for recycling to be used again. I know that because I worked at a scrapyard/recycling center for over 2 years. Check your MAP or MAF sensors. Typically, older model Jeeps use one or the other, and rarely both. I have a MAP. That's it. If your MAP is malfunctioning, it would cause ignition timing to go crazy. I know you said you replaced it but get it with the voltmeter and see what it reads at different times. It may not be the sensor but the computer, misinterpreting the voltage. Our MAP's are usually 2 wire configurations. 2 wire has a shared ground with a set 5v control voltage from the computer going through a resistor to the sensor which has another resistor then going through to ground. The way this changes is as the resistors change Ohm readings, the computer translates the voltage changes into set manufacturers values. The computer ALWAYS sends out a 5v reference voltage. If it doesn't, you got major problems lol check to see first that it even sends out voltages to your sensors. They should always be 5v controls with a variable rate between 0.5v-4.5v. If something isn't getting that reference voltage, it won't work. Check for those then see if they are wrong. If they are, clean all your grounds for the computer and the affected sensor.