You didn't mention replacing the cable inners or checking the functioning of the shift mate. I worked on a tandem recently where the travel agent (same idea different cable) was gummed up and not working properly. I am assuming that your new cable housings are finished nicely as per Sheldon.
The other idea I have is if this is a 10spd setup, have you used a road cassette? Shimano (the people who brought you Positron, Biopace and 10mm pitch chains) have made their MTB cassettes to be slightly different to their road cassettes, just to be difficult.
A SRAM rear derailler will do the job.
SRAM levers pull the same cable amount as Campag ones. SRAM cassettes are the same as Shimano.
Zinn wrote an article about it some time ago.
Cheers,
Cameron