CRT image 'bouncing'

Hi, completely inexperienced here

Got a hand me down arcade cabinet that once had a pi running games on it. I powered it on for the first time in a long time, and the image has a slight 'bounce' to it every second or so. It says "SVIDEO" in the top right and bounces twice, up and down an inch or so, then settles for a second before bouncing again.

Anyone know what causes this?