Update: -

Cable from BDP83 and 990 replaced with a 2' HDMI to DVI cable.
Run from 990 to projector replaced with a single 20' DVI to HDMI cable.

Problem solved! laugh

So it appears that the HDMI to DVI adapters were causing the issue.

Thanks for the input gonk.
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Outlaw 990
Outlaw 7125
Oppo BDP83
Panasonic PT AE4000 projector
Linn Keilidh mains
Linn AV5140 center
Royd Coniston R rears
Custom vented sub (2 x Dayton RS390HF)