Besides coils/plugs and vac leak, what causes misfire and "too lean" codes??

For months I've had random misfire problems and engine too lean codes. I changed coils and plugs and it was better for a day before engine light came back on. Rough idle, low power, mpg loss, light will flash when trying to accelerate at higher speeds. Changed PCV valve and hoses after mechanic found small vac leak. STILL same symptoms persist. Mechanic supposedly checked MAF sensor and other easily accessible potential suspects. Idk what to do. Is it wiring to coils? Fuel injectors? Fuel filter? 2 mechanics so far have tried and failed to find solution.
2003 Navigator 2WD 160,000m 5.4L