My husband is working on his 2004 RX8 right now. He has changed the spark plugs, coil packs and spark plug wires with all new thinking that was the problem after it died on him coming off the interstate. It did start up but now it wont idle, still shuttering and missing. Sounds like its miss firing. What else could be the problem?

 

Lots of things cause misfires; how many miles are on it and what kind of transmission does it have? Other then the tune-up, what other work has been done to it?

 

Did he get the coils in the right order?
Did he get the plug wires in the right order?
Has he attempted to clean the MAF?
Is the CEL light on? If so, need to go get it read.

If he just did all you said......did he unhook the battery to do any of this work?
If so, then did he do any of the resets?

Have him also check that the plug wires are on good, they should sort of snap or click when put on right.
The front leading is known to not sit real well and can pop off.