Like so many, I've not had much luck decoding PSK31, even with overclock on high (950 MHz), and even with lightweight apps, much less FLDIGI. Using the latter's WWV calibration, the one-second blips were almost a horizontal line rather than a vertical one, and timing between them was erratic.
Fortunately, today I came across Amateur Logic TV Episode #53 where they took on the question, "Will the Raspberry Pi decode PSK31?" Using a stock Pi, they found FLDIGI a bit too resource intensive, but had success with LinPSK, using the guide at the A Slice of Raspberry Pi blog.
They made the point it was critical to have the USB soundcard plugged directly into the Pi and not through an external hub. That was my problem--the latency of that hub! Like so many, I've gotten used to connecting everything to a powered hub to save power drawn from the Pi.
I jumped up and rewired my setup, plugging the Griffin iMic (that connects to Line In/Line out of the Elecraft K3) directly into the Pi. I fired up FLDIGI WWV with the K3 on 10MHz and got an almost vertical line. A correction of -90 made it literally vertical. So I QSY'd over to 14.070 and reset the FLDIGI modem to BPSK31. This being Field Day, the waterfall looked like the upper levels of Missile Command. The "Signal Browser" was showing strings of mostly correctly decoded contacts. I bumped the squelch on it up to 6.0 to keep the number of streaming contacts down to 4-5 and copy got cleaner. At this point, overclock was on "medium" (900 MHz, 2 overvolt). I bumped it up to "high" (950 MHz, 6 overvolt), rebooted, and copy got a bit cleaner.
FLDIGI is running 55-65% cpu utilization, and Xwindows is taking 25-30% feeding a 1280 x 800 HDMI monitor with all this motion. I've yet to see "top" report 100%, but I'm sure it's hitting that sometimes. gMFSK (which works well with xLog), uses about half the cpu as FLDIGI, but if I can keep it working, I'll stick with FLDIGI. On mostly less busy days, I'll stick with "medium" overclock.
So, many MANY thanks to the guys at AmateurLogic.TV! This really rocks!