Sudden jump to the left

(My Captivate 7 e-learning is 800x600, has a custom theme and has "rescalable html" selected.)

The first few slides run fine, with the e-learning in the centre of the browser window, then a slide has a radio button interaction, and as soon as this interaction appears, the whole thing jumps over to the left hand side of the browser window.  I remove the radio button interaction and it doesn't do this, put another radio button interaction in, and it jumps again.

It does this only in the published Flash version, not html 5; and it doesn't do it in the F12 preview, only when published and sitting on WAMP server.

It stays over on the left hand side of the browser for the rest of the e-learning.

Functionality all  works fine, it's just very, very ugly.

Has anybody come across this or know how to stop it happening?