Two answers:
1. as Urbanknight said, Shimano brifters will gunk up with old grease over time. You need to flush it out with WD40, and then re-lubricate with Finish Line dry lube.
2. The Dura Ace/ Ultegra/ 105 10 speed brifter, even in the best of time, was vague feeling. Because the amount of cable pulled was so short, it's very sensitive to cable wear or friction. Make sure you have new Shimano SP41 housing, and slick type cable. Greasing the cable helps too. Especially the tight loops.