[Netspeed]

Here's something interesting that happened last weekend that is like your problem:

Some friends were running their Alfa 2.0 at the track and the thing stumbled but more at higher rpms....they went through everything. Replace all the high-voltage ignition stuff and then ruled that out. They changed the jets and eventually removed the carbs to inspect. Nothing wrong....guess what it was? A poor ground at the coil. Although it was bolted to the firewall, it was corroded enough that a the system produced a weak spark.