I actually had the opposite problem: revs would drop like a rock, especially between 1st to 2nd gear shifts. The result, for me, was a very intermittent engine stumble when I let off the clutch pedal during casual, around-town driving.

I had been to the dealer several times about it. Each time, the mechanic was unable to replicate the stumbling problem when I went on a test drive with them. I was convinced that it was a programming problem, but at this point the car is out of warranty. (Be aware that my car uses the drive-by-wire, electronic throttle control system. Hence, my assumption about a programming problem.)

Last week, I was considering IPD's ECU upgrade as a possible solution to this drivability problem. It would be an expensive way to fix a problem that may well have been designed into the car, but I thought it might save me the trouble and expense of trading up to a car that drives properly. While browsing IPD's site, I found this:

http://www.ipdusa.com/pdf/wastegate_adjustment_94_02_turbo.pdf

I tried it this weekend.

Interestingly enough, my wastegate had the base setting at 3 psi, rather than the 1.5 psi that IPD thought that it would. But the adjustment is easy enough, so I tried kicking it up to 4.5 psi. This seems to have increased part throttle torque and responsiveness, and *I think* that I'm also seeing the revs drop more slowly than they used to. I haven't yet had one of those annoying engine stumbles, although it's a little too soon to be sure that it's gone. (Fix on Saturday -- it's Monday as I write this.) But the wastegate adjustment seems to affect the rev drop between shifts as a by-product of the base-boost level.

I bring all of this up to encourage you to try a similar adjustment. Get a pressure gauge and a hand-pump -- a "Mityvac" hand-pump works well for this, see http://www.mityvac.com/ -- and get a baseline measurement for your current wastegate "base setting." If it's up considerably higher than the 1.5 psi that IPD says it should be, or the 3 psi that I found as a base setting, then try LOWERING it with the locknut and adjuster. Hopefully, it will have the opposite effect of what I was after (and may have attained).

Good luck with it. Please let the forum know if this helped, because you're not the first ones to post about such problems.
--
Jim Rothe, '99 S70 T5M,
http://www.jimrothe.com/volvo/index.html