engine is misfiring or only firing when it wants to

It could be so many things. Replacing parts without absolutely proving they are bad is rarely cost effective. It could be electrical, you would first need to determine if you are loosing spark or fuel when the engine fails. A lab scope hooked up to key engine sensors right back at the computer would test both the sensors and wiring. You would need a wiring diagram for the computer. Autozone have some popular workshop manuals for free on line once you register (for free) on their website http://bit.ly/autozone_repair_info. There is also a company that independent auto repair shops buy their information from that the same information inexpensively to do it yourselfers. The information is year make and model specific repair procedures, service bulletins, component locations, wiring diagrams ect.... Alldata is very easy to navigate http://bit.ly/AllData_Repair_Manuals_Online Alldata have good test procedures, easy to follow.