1999 Pontiac Firebird loss of power issue

Hello, I read your article on noises and possble causes/solutions. I could not find anything that matched my situation though. Allow me to start by saying this problem started after replacing the EGR valve, due to a trouble code indicating it needed to be replaced. After replacing that, same day, I was driving at low RPM(~2500 in 4th gear) and my rpms dipped suddenly. This happened a couple times. Then the next day I was driving at high rpm and the power kind of platoed and would not allow me to accelerate any higher. Today I was driving and noticed a hissing/maybe scraping noise caused only by accelerating and a huge loss of power. I had a long ride home to try several scenarios. I tired downshifting/release throttle to see if the same noise occured. No noise. I tried maintaining a constant speed or accelerating very gently. No noise. I tried shifting to neutral and reving. Couldn't tell over the engine noise. When I got home I checked for leaks around the EGR valve and didn't find any. Got as close as I could to exhaust pipes and couldn't hear anything except for something new. When I was at the exhaust tips I noticed a "pop" noise ever few seconds inconsistantly. I was almost convinced this was an airflow issue up to this point and now i'm thinking it me be ignition related or maybe something else. Suggestions? Your help is much appreciated. I wish I could donate more, but i'm broke after buying that EGR valve.