2002 DeVille with rough idle
 

I recently purchased a 2002 DeVille with approx. 48000 miles on it. I noticed a rough idle. I took it into the shop to have it looked at. They told me it needed a tune up, new plugs and boots (not covered by warrenty) that helped a little but not totally. Then they replaced the coils. That took care of the misfires on 5 of 8 cylinders (covered). Now they want to look at a gasket or intake valve. First what should they guys be looking at and my second question is would the plugs and boots be effected by the bad coils and possibly intakes and if they are shouldn't they be covered by a warrenty then and not chalked up to normal wear and tear?