I have ano 04 Bonneville gxp, rebuilt jasper, new coil packs, new plugs, i keep getting a random misfire code. Car doesn't feel different, but when I'm on the highway getting on it (80+mph) the service engine soon light will flash for a moment, then come on steady, no difference in feel at all tho. Light shuts of after 4 or 5 key starts then all is well until I hit the highway again. Any suggestions of where to look next?

Don't you know the speed limit is 65?

Try this, get on the highway and find a slight upward grade.....cruise at about 50-55 up the grade, and then slightly accelerate....does the engine misfire under this kind of load?

Naturally, riding with a scan tool showing which cylinder is misfiring, is the optimum way to find the misfire....

When did the misfire start? after replacing the plugs, coils?

Could try a coil output test with an ST-125....

How old are the wires?