[wiswind]

The vaccum to the EGR valve, when the PCM calls for it to open, will be MODULATED.
The DPFE will measure the EGR flow....and the PCM will use this measurement to modulate the EGR valve to match the DPFE reading to that value that the PCM is looking for.

IF the EGR valve is stuck, I would expect an error code indicating a EGR flow as measured by the DPFE, when it is not being called for.
Perhaps a DPFE is getting marginal?....but that is only a guess.....but as they are somewhat a high failure item (more so than most any other EGR component.....not counting clogged EGR ports), you might try replacing it....and you will know for sure.
This is all sounding like something is marginal.....not totally failed.....and that is what is making this a hard problem to track down.

Of course......you know....but I mention for anyone getting the message from a search function.....
Disconnecting the vaccum line from the EGR valve......disables the EGR valve, so you should get a CEL, and a code indicating insufficient EGR flow.....as the PCM is not able to open the EGR valve.