94 camaro
Digital EGR valve
3.4 SFI
automatic
Service engine light comes on  going down the highway when I let off
the gas.  3 codes for all three EGR solenoids.

I have pulled the EGR valve off cleaned and tested it with 12 Volts,
all three solenoids work. Removed the EGR tube and cleaned it,
replaced it with new gaskets, made sure hole to the valve is open.
Sprayed the tube with WD40 while the engine was running, no change in
the idle (no leaks).  With the key on and a wire from the 12 V on the
EGR harness and shorting each solenoid to ground works fine.  With the
engine running and shorting each solenoid there is no difference in
the idle of the engine.  

Could there be a blockage in the exhaust manifold?
Could there be a blockage in the intake manifold or a sensor that
detects whether the EGR opened or not?
Car runs fine, just tired of seeing the service engine light coming
on.
I know how too disconnect the service engine light, but thats like
taking the battery out of the smoke detector.
Any leads on what else to check, would be much appreciated.
Thanks