Ford Repair: check engine code 335 336 337, vacuum gauge, oxygen sensor

Question
I have a 1994 Ford Explorer 4 liter v6, auto trans.  The check engine light comes on intermittently.  The fault codes are 335, 336, and 337.  I have replaced the oxygen sensor and  the EGR valve but still getting the same codes. What do you think I should try next.  I am considering the EGR pressure sensor or the EGR vacumn solenoid (spelling?), but I am not sure which one to go with.  What is your opinion.

Answer
Hello and thanks for the question...I'll try to help.
What I would suggest is that you perform a key on engine running test and have a vacuum gauge hooked up to the line that goes to the EGR valve.......see IF it's getting a vacuum signal at all.
Erik