Hi John
it'll probably be diffraction from something intruding into the light column (screw, spider support, OTA edge etc). Maybe try back- projection if you are having trouble isolating it - you will soon see if there is anything amiss. Put some translucent baking paper over the entrance aperture and shine some light in backwards through the scope from the centre of the focal plane - the shadow pattern on the paper will pick out any intrusions.
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Centering the light column by adjusting the secondary in-out can also help reduce diffraction patterns on the edges of the field - back projection can help you do this, but you need to do the following sequence: collimate scope> check if light column is centred> if not, adjust adjust secondary in or out a couple of mm> collimate scope> check if light column centering is better or worse> adjust secondary more if improving, reverse direction if it got worse> collimate scope> check light column centering.....etc