I've replaced: air filter, thermostat, both O2 sensors, EGR regulator, Coolant Temp Sensor, New Plugs [8], new wires [8]: After driving approx 15-20 minutes, steady acceleration from between 40-50 MPHr - knocking/pinging occurs. Also, when accelerating, the temp gauge needle falls back and then recovers??? Spending lots, gettin' nuthin!!
Have a new fuel filter in hand. What next? HELP! Thanks a bunch.

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1998 2.5L XLT 163K miles

What grade gas have you been using?

Have you tried cleaning the MAF sensor?

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Thanks for the quick response. I failed to note that I have run one full tank of fuel, [87 Octane 10% ethanol] with an injector/fuel line cleaner. I have used a CRC Electrical Contact Cleaner spray on the Mass Air Flow sensor. I may have been overly expecting, but it did seem to make a difference, [for the good] temporarily. Will try your recommendation.
Some people from work have also mentioned a potential vacuum leak and that I spray starter fluid around the vacuum lines/connections with the engine running. If a leak is detected, the engine RPM will increase at the leak source.
Thanks again.

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1998 2.5L XLT 163K miles

I was gonna suggest checking for vaccum leak if you had tried the others with no results

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