Y axis jittering on first layer

Hi all,

When laying down the first layer, my printer seems to jitter in the Y axis, causing there to be gaps in the plastic.

Oddly it seems to be the first layer only, and when filling large areas (perimeters and skirt print just fine).

I've checked the axis is free, adjusted the current pot up and down a little, checked there is no binding in the belt and nothing seems to be getting hot. I am suspicious that it may be software related as it seems to happen in the same place each time.

Any clues?

Many thanks in advance, Phil

Check belt tension.
Check that pulley is well fixed on the motor shaft.

Where did the third perimeter on the left side of the picture disappear?