The owners manual is the place to start. With that many miles it is just about everything. If you do not have the owners manual you can find it here on the website in the TBS and how to articles section, look for the "index of crossfire problems".
When you replace the spark plugs get the special tool, it is available on ebay, which makes the job a lot easier.
The one upgrade you should do to help your engine is an oil catch can. Needswings makes a good one.