I had a problem with my machine surging and I was coming to the view that the pump was going. At first I thought the grind was too fine and that's why the pump was struggling, but backing off the grind just made dirty water.
After looking at the posts and videos on the new Decent Espresso machine, I suspected it was channelling. The graphs that show up on their machine demonstrate channelling (pressure drops, channel closes, pressure goes back up) - although it is micro because they have a better grinder than my Breville. They made the comment about the quality of the grind, tamping and pre-infusion. Their machines pre-infuse at low pressure for up to 12 seconds.
I tried doing that manually on my 6910 - let it pump for about three strokes (making sure I primed it before putting the portafilter on) - let it sit for 5-10 seconds and then run the shot.
Without a change in anything else (same grind, same tamp), I now have no - or very little surging. It is probably placebo, but it seems to taste better too.
The other thing I do is I do is make sure the grounds are distributed evenly across the basket by stirring with the thermometer probe as the grinder drops the grind into the basket and then try to even out the grounds.