I had a brother 3100 (ULT2003) do the same thing. With help from Ann's fix it page I was able to fix it. If you look under "Foot sensor" my sensor arm had come loose. It was a tiny grub screw that needed tightening. Have a look, it might be something you can fix yourself. good luck. xx
I have a 2003 ULT, but I never knew it was a designated "3100" - learning after owning it for 9 years! Thanks for info, lyns.
And BTW, Ann's fixit tute is a great 'do it yourself' for this machine--prob other models also.
I wonder if it's related genetically to my Juki(sewing/quilting machine),LOL. When I use the knee lift it does the same thing. I've come to realise that the knee lift has come loose in the socket and doesn't return properly, so if I nudge it back to the left, it behaves :)