How did this turn out?

My 2001 wrx has just started doing the very same thing as you described in your first post...

This has been solved! I took it to be remapped in the hope the experts would know what was wrong. Instant cure! The wastegate actuator rod adjustment had been tampered with to increase boost. The ECU has an upper limit for boost which if exceeded it reduces boost. It then allows it to build again, it over boosts and backs it off again. The fix was not to adjust the rod but to remap the ECU for more power and higher boost levels. My otherwise standard car now has 270bhp and pulls really well without surging.
Try slackening your rod (ooh-er!) and seeing if that cures it.

Ah,
Good to hear you fixed yours, but, I only noticed it on mine after picking it up from a remap! I sincerely doubt that it was the mapping guys as they are very reputable and professional.
I was just assuming that it had only become obvious due to the now higher boost.
Haven't even started looking at it yet, only got home an hour ago. And it's dark. And raining. And bed time!
I'll look into it tomorrow and start by talking to the company who did the mapping.

It should show up on the dyno and easily be identified. Racedynamix dude did mine.