Help diagnosing rough idle...
 

I was gifted a 2003 Cadillac Deville, though it has some issues I am trying to address.

Currently it has a rough idle, it definitely feels like it is misfiring (almost like a "hiccup" within a couple of seconds of each other). Once I apply gas, it smooths out with no issues. I have cleaned the mass air flow sensor and replaced the spark plugs. But no luck. When I replaced the spark plugs, it had a smooth idle for about a day, then back to being a rough idle. At this point I am unsure of what step to take next. I have an Ignition Spark Tester, but seeing as I have to remove the entire ignition coil for the front 4 plugs at once, I am not sure if this is a safe way to test each terminal.

Can anyone help me figure out what to do next?

Thanks!

Coil packs are expensive but that's the next step imo.