Sport trac ignition problems
 

I have a 2007 sport trac 4.3 liter (134,000 miles) that fills like it is mis-firing when I reach about 45-50 mph, 1500 - 2000 rpm's and has shifted into high gear. The porblem is not bad enough for the CEL to come on. Its just annoying. Coils have been replaced, plugs have been replace, what next, fuel injectors? Anybody have any thoughts on this. Transmition was replaced 4 months ago.