I have had same problem with 749s. This thread started by me went into some detail about what 100% Bikes Suspension thought of the 749.
DSC Thread - 749 Showa rear shocks I was quite frustrated when I tried to set up the rear suspension to the "ideal" sag figures. I found that at about 15stone I was getting sag of 50mm or more and in riding I was experiencing a pogo like effect.
The bottom line from that other thread was that spending £40 with Colin was well worth it. He turns up at various trackdays, but you'd ahve to phone him to find out where he will be. He said - the rear on a big Vtwin can be a bit softer because you need it to damp out the power pulses. The first thing to set is the Rebound damping. Section8Superbike have some good info on how to check and adjust rebound and compression.
section8superbike - suspension tuning My Showa has ended up with virtually no compression adjustment left. He did this do try and compensate for the shock not doing well for track use (details in above thread). Whether that is right or not, it now handles FAR better than it ever has, and I tohught it was good before. It now feels totally stable and turns 'on a sixpence'. All in all, the best £4 I ever spent on it.
[Edited on 4-10-2005 by HW]