Yeah, I had that problem as well... something's gotten loose, or your jack may just be a pretty lousy one. It's a common thing - mine sucked as well

Get a tech guy to look at it if you're unsure about pulling it out to check for loose wires, though it may very well be the case that the actual mechanical structure (i.e. the thing which keeps ur jack from moving side to side) is damaged/worn out as well.
Good luck with it!