If the "power loss" you are talking about is a hesitation or surge, and it is during crusing speeds, my first guess is your EGR valve. The EGR valve operates under around 50% throttle, and if the diaphragm is shot, it will attempt to operate, but will cause a power loss and a sort of surging motion. Otherwise, make sure the basics are covered: Plugs, wires, cap, rotor, fuel filter.
As far as reading codes, you can do it yourself an save a decent amount of money. Check out
http://www.cfm-tech.com