94 trooper check engine runs bad

I have been dealing with the check engine light on my 94 trooper ...it seems to come on when on the highway then off when i get off the highway...once it does come on my truck runs bad going up hills or when i give it gas past aproximatly 1/2 way to the floor.....my mechanic says the code keeps coming up lean exhaust
changed cat converter
changed EGR
cleaned injectors
changed fuel filter
cleaned maf sensor
changed o2 sensor
checked fuel pressure

still hasent fixed the problem.....