When my EGR went it threw a code, but wouldn't hurt to try one if you can get your hands on one cheap. Try a wrecker and see if you can snag one fairly cheap and throw it on, before you spend big money on one from a parts store, as usually it should throw a code if it's bad.

I would also check all the hoses for cracks in and around that area, is you said it worked better when you put the blocking plate in, so possibly you could have moved something around (?).

Also check your cat when the car has been running, just take a look and see if it is glowing red just in case the cat is plugged, may not be but for the five seconds it takes, no harm in taking a look

 

The older 3100, like my 94 grand am had, used an analog get with 3 solonoids. Those had some issue with carboning up on the inside. The newer digital ones are more reliable.

Your motioned you are using a snap on scan tool, is this one that also includes functionality testing? If so, you can use it to control the egr, while idling increase and decrease it's value. You should notice a change in performance (if I recall, the engine may begin to bog some, but it confirms the egr is responding.

My guess, if you noticed blocking the egr and problems went away either the egr is not responding to the PCM properly or something is wrong with the exhaust flowing into and through the egr. This is just going by your egr observation.

 

Thanks for all the suggestions everyone! I still can't find anything wrong with the car, but I'll keep looking.
I'll update when I find out what's wrong with the car and what fix I performed.
Thanks again!