03 misfire under load
 

I have a 03, 4.6, 5-speed 4x4 with 79,000 mi. It has had a contiinual misfire for about 9 months now. It missfires mostly when at low RPMs while under load. It produces no codes. I have replaced the fuel pump and fuel filter. I Replaced coil #4 at 48,000, and the idle air solonoid at 38,000. The plugs were new at 48,000 mi in 2005 and they are platnims. I borrowed a friends dianostic reader and I found when the missfire occurs the low fuel trim goes negative and the timing retards. I have put off this problem waiting for a code to come in, but this problem is getting worse so I need to address it. A side note when outside air temps were below 25 degrees the problem vanished... I don't have a clue where I should start looking. Any thoughts or direction would be greatly appreciated..