Time Machine stalled for days

Time Machine has been working OK for me until recently, when it seems to have fully stalled. I was running out of space on my laptop so I off-loaded about 150GB of media files, most of which were large in size so the actual number of files was not terribly high. When I did the first TM backup after that it worked slowly, then after a day or two of running full time finally seemed to stall altogether with the notice "Backed up: 66.83 GB." After a day or so stuck at this point I had to shut the machine down, so when I restarted I launched Time Machine again. After a while it get back to the "Backed up: 66.83 GB" point, but now it's been stuck there for about two days straight (again, I've left the laptop on overnight with no other program launched). The Activity Monitor currently reads: backupd - %CPU 1.6 / CPU time 6:25:04.49.

Any guesses?