occasional loose stitch when sewing

[SonderingSusan]

 

Hey everyone , 

i am attempting to sew a seam with a right sided foot ( if that makes sense) so the seam will be right up to the needle of the foot. 

Its not the usual foot i use for such  job, and after hours of tinkering with everything i am finding that there is STILL about one kind of loose stitch every 30 stitiches.

I have included an image of it below. 

All / any suggestions about what I'm doing wrong or what i should check would be very appreciated. 

thanks in advance!