Hmm, I thought my pump was on the blink the other day, but I've just had the same thing happen, and with the SM decaf blend again.
I've been grinding quite fine with this stuff as I over-roasted it rather (no 2nd crack it seems). When I pull a shot I get quite quick flow for about 10 seconds (after 5 seconds pressure building before flow) and then it stops . When I release the prtafilter it is very soggy on top. It seems that after a while the puck becomes more resistant to the water and the flow stops, how wierd is that?
I can only think that there is some strange effect with decaf beans that the grounnds swell more, and restrict the flow after a while.