My 123gt(b18b, twin SUs, manual trans. with OD) is bogging down a bit when taking off in first, and stumbles at the top of the rpms before each shift. It really stumbles when I try to push it past 60 mph, and very occaisionally backfires at highway speed. This has come on quite suddenly, it was running very well a week ago. I assume it's either the ignition or fuel system, but any clues about where to start looking would be appreciated. It's cold and snowing up here again, and the less time spent in the driveway figuring things out the better.