Intermittent rear window after regulator replaced

I've just replaced the window regulator on the right rear window (cable snapped in old one), and now the electric window is randomly not working. It'll go up and down no probs for a bit then suddenly pressing the button gets no response. Can try for 20-30 seconds and get nothing, then suddenly will work again. There's no sound when it doesn't work, so not sticking. Tried front and rear switches and happens with both, so not switches.

I did briefly try it out of the door and SEEMED to work fine, but I did only try it briefly so don't necessarily take anything from that - I just assumed I'd ballsed something up on initial install and put it back in the door. I would have took it back out the door and given it a good long try out but was out of daylight, so decided it wasn't worth ballsing anything else up by working in torchlight.

Anyone got any clues? When it's working it works fine, no hint of motor struggling. Just randomly when pressing the button nothing happens, and won't do anything for 20-30 seconds, then works again.