I had the same problem. What you need to do is adjust the EHA, but then adjust the mixture in the air intake. They need to be in conjunction with each other. I think to set this properly you need to test the fuel pressure at the injectors?? Ive only ever adjusted the EHA screw and trialed it. I got huge improvements.

Eha's need to be adjusted when new, its a simple fact.

Oxygen sensor can also be tested.

Test the output wire which is under the drivers side carpet and is green. Put the -ve test meter wire to earth. Test for volts and it should be between .1 and .9. .9 being rich and .1 being lean. If you rev the engine slightly it should fluctuate.

If it does not fluctuate then theres problems and further tests need to be done.