Nigel,
This sounds to me like the sync signal coming from your DVD is not powerful enough to supply a stable picture. DVD players don't normally output a pure sync signal, so the sync from the composite video output is used instead. This signal really needs to be isolated and amplified to provided a reliable signal to the projector. This can be done with an LM 1881N chip and some fairly simple DIY electronics. Try here for general details:-
http://www.hut.fi/Misc/Electronics/f...rgb/scart.html
You should be able to build the unit for under £30. All parts are available from Maplin although you may have to order the chip.
I have built a couple of these amplifiers for friends who have Sony 1020QM projectors with excellent results. These projectors do not have scart on board (I did not know the 1031 had). The amplifier was used in conjunction with RGB scart to 4 BNC connectors R, G, B, combined sync.) Without the amplifier the picture would roll in and out of sync but now images are rock solid. Perhaps you might want to consider a change to this connection method.
best of luck
Alan