Got a new sewing machine today (Brother 3600, from Spotlight), and can't work out what to do about this problem. Is it a tension issue?

When I sew the top stitch is fine, nice and even, no skipping or looping, but the bottom stitch from the bobbin is terrible - bunched up, looped and and tangled

What the? I've played with the tension, and sometimes it works but then it just starts up again

Any ideas?