95 Altima GXE pesky check engine light

I've had a check engine light on for a couple of weeks now. (95 Altima GXE) I had the car scanned, and 2 codes are persistantly coming up--EGR malfunction, and knock sensor. I cleaned out the EGR valve & hoses(the valve seems to be working just fine, but it is not getting pressure sent to it for some reason) As far as I can tell the Backpressure transducer (BPT) is also working fine. This morning I swapped out the EGR control solenoid for a junkyard part--this had no effect on the problem. What am I missing here? Thanks, Phil

my car when it runs 3 hours may suddenly stop and every effort to get it to start always proofs abortive, but after leaving it overnight it eventually started and this has been the same for 2months going now.