Many thanks to all of you.
I think the head cleaning has to be the first try. I don't think that the capstan is snagging, 'cos other tapes run through with no problems.
Big_J the tape is a camera tape, not VCR, and I only have access to the one camera.
The thing I find most puzzling is that the pixellation happens at random, varying with successive playbacks
Conan - the original problem may well have been down to humidity/temp change; the camera had been in quite cool storage for some time previously.