I think this will be the standard problem with the Bosch 5.7 unit. It gets heat soak and will be fine while cold then stop working after maybe 10 mins of so, the speedo also stops with it.

It is a well known issue on cars with this Bosch unit (BMW) etc there is a detailed thread on here I think. If it has not gone to far it can be fixed, the issue for us is that the software is MG/Rover V8 specific and no-one has it I believe. So buying a new or used 5.7 unit off ebay off another car will not work long term with our car, so your 260 one needs to be fixed.