You're right to practice before canning anything valuable, canning water is always good so you see if there's any leaking going on etc.

Hopefully others will speak up too, but I would ask if you are sure that it was vented properly, assuming it was vented for the correct amount of time (10 minutes for mine, but your instruction book may differ) then it sounds to me that it is boiling too fast.  It takes some time to 'cool down' (a relative term of course).

While it is venting, you want to get the burner adjusted so it is blowing out steam steadily, but not at a full on rolling boil!  More than a simmer of course, but less than a freight train would need to get up a hill.  It can be a subtle difference, so take the time to get a real feeling for the correct level.

Most of the weights will only give a periodic bump or jiggle, not a fast constant jiggle.  More steam is not better, as long as the pressure is reading steady it's good!  That one we tried at the retreat didn't actually jiggle much at all, the 'bump' was barely noticable.  Learn how your canner acts is my advice, those jars of water are good to have on hand too you know.