The egr can provide an idiling problem if it is open at idle, but since they blocked it off that shouldn't be your idle issue, all though make sure they fix that problem because you won't pass smog and your check engine light will never go away since it doesn't see your egr valve opening or closing. As far as the idle dropping there can be any number of reasons for that happening, first thing I would do is go back to the shop force them to fix your egr or tell them your going to the b.a.r. and reporting them for tampering with your emiission systems. Then take it to a smog repear station and tell them to properly check your egr system to make sure it's fuctioning properly because if it is open at idle it will drop the rpms possibly kill the engine or sometimes create a stumble on accell. If that doesn't take care of it maybe you have some dirt on your idle air control valve and it's causing it to stick or something. Does the idle drop and then come back up, or does it drop and stay down?