Hard to tell without being in the car. The shuddering may possibly be a misfire if it's only noticeable in Park or Neutral - do you also get slight jolting under acceleration?
Changing the plugs is easy.. if you know what the basic parts of an engine are, there's nothing different about these engines. You have 6 spark plugs, lined along the engine, with wires going into the top of them. The air filter is inside the big black thing.
These are marked AF and SP on my 214,000 miles 300TE's picture.
By the way use basic copper plugs, not Platinum ones - these can cause all sorts of problems in these engines (do a search).
But if you *really* need this help, I would get someone else to show you how to do it first.
2.5-16v 1990 90,000m Astral Silver
2.0E 8v 1986 107,000m Black