I have a 2001 xc70 with these exact symtoms. I did have the ecm computer upgrade and it did help for a while.
Now it is doing it again. Few things I am going to check for ( when it gets warmer out) are
1. EGR valve, I am going to remove it and clean it ,make sure the carbon is removed and the valve functions. A faulty egr can cause surging.
2. pcv system, going to go through the whole thing
3. I have an oil issue and I believe it is from a clogged turbo oil return line.
4. check all vacuum lines, vacuum leaks can cause surging.
5. Use throttle bore cleaner and clean the throttle bore,etc.
6. Get new spark plugs and gap them.
Hopefully with these above repairs ,it will solve the surging