tellyho--I guess you've had success then with using a SRAM shifter (one intended for their own derailleurs such as the x-series) with a Shimano derailleur? The derailleur does have something to do with it...

OP--I'd try the old derailleur first, can always change that easily enough if it doesn't work. Here's a good article if you haven't seen it http://www.sheldonbrown.com/speeds.html