It's Scrobble that randomly stops. I have that one installed, and I have never even looked at Scrobbled.
Last week, I noticed that I wasn't scrobbling anything, and I fixed it with a respring. I've had it on my iPhone since I first jailbroke (it's been about three months now), and this is the first time that I have noticed it.
This is with very heavy iPhone use, including music daily.
I think I may try Scrobbled for comparison.
I'm curious though... With both installed, don't you get double submissions on Last.fm?