if your going to do the gasket replacement just wait on the thermostat, because your going to have that apart anyways. its not the easiest thing to replace, so just wait until your going ot be in there.
i had a vacume leak once. the hissing was crazy, and the car ran like crap. i forgot to clean the UIM gasket off the LIM. caused a bit of a leak.
as far as rough idle that could be anything. i would start with plugs and wires, if cleaning the buttlerfly in the throttle body, and maf doesnt work. if you even have a miss its either plugs, wires, or coils. atleast thats been my experiance in the past 8 years and 100k + miles.
Too late on the t-stat. I replaced that, and the housing last week - along with my radiator, water pump, and hoses. I did the spark plugs and wires in June, so unless they were faulty from the start, those are ok. But, I will inspect them to be sure.
And I completely forgot about the ignition coil. I'll be working on the car again tomorrow. I'll let you know if any progress was made. Thanks, metal.
also fyi if you do pull a coil there are (2) 5.5mm bolts. one on the front, and one on the back of the coil. i know 5.5mm sounds really dumb, and it is but thats what it is.
Alright. I did the cigar test and the issue was pleasantly simple. The lower lip of the air intake boot was not attached to the throttle body. That was on me. After I did that all day job last week, I was in a rush to put the car back together, and I didn't attach the boot properly. I'm a very happy man right now. I cleaned the throttle and MAF sensor just for good measure, though.