As said above it is most likely to be the thermostat, they are very common failure with the straight 6's. They usually fail open so the only consequence is overcooling, but I have known them buckle and fail in the closed position.
Just a point to note, when my t'stat failed I went to the local motor factors to get one and asked for a 'stat for a 960. They asked me for the engine size to which I replied it doesn't matter as they are the same. Their computer system had different prices for the 2.5 and 3.0 versions, so I asked them to check the 850 and S40 versions as well because the whiteblock series of engine use the same stat. They came up with £3.29 for the S40 1.6, £3.59 for the S40 2.0, £3.89 for the 850 2.0, £4.29 for the 850 2.5 and 960 2.5, and £4.59 for the 960 3.0.
I got them to check the catalogue for the part no. to confirm it was the same part and then asked for a 'stat for a S40 1.6!
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