Ford Focus Mk1: Instrument cluster erratic behaviour, with (or without) internal/external lights issues (Apologies for verbosity)
This first occured approx 4 years ago. Whilst driving on the motorway (typically I had to be above about 50mph) the speedo sometimes drops to zero, then returns to the correct speed a second or so later. This may then be followed by the rev counter dropping to zero and a second or so later recovering, then the engine management light coming on and going off, the odometer display going to horizontal lines and so on. Sometimes this would be completely at random just happening every now and then. At other times it would happen for several minutes with the dials and warning light going on and off though usually not all together. I found a few forum threads and implemented the removal and WD40'ing of the instrument cluster but this had no effect. (A friend had had a similar problem on his Focus and had found that this cluster connector fix had worked for him **). As the car drove perfectly well with no mis-fires or other safety issues I continued to drive it while monitoring whether the problem was getting any worse.
However, after a month or so it then progressed to another level. As well as the instrument cluster issue it would then have periods with all the vehicle lights flickering including the headlights, internal lights and all of the console illumination. Having found another forum thread, and taking a bit of a punt, I had the battery changed but also had all the earth bonding points cleaned and tightened. All of the issues went away (but it was unclear whether it was a battery fault or the earth bonding). However the important point was that it was nothing to do with the instrument cluster itself but was the primary electrical supply to the vehicle.
4 weeks ago the faults re-occured. This time all of the symptoms started at the same time. Instrument dials dropping to zero, warning lights coming on and off, internal/external lights flickering etc. In a series of tests it seemed that the issue became worse if the A/C was switched on and/or the headlights, heated windscreen, heated rear window or other heavy electrical loading. Again this pointed to an electrical supply/voltage problem. After inspection, I determined that the negative battery terminal was slightly loose - not really loose, you had to put some pressure on it, but it could just about be turned by hand. Also there was some corrosion on the earth bonding point from the neg terminal to the vehicle chassis - this point is located under the air filter box. I cleaned the bonding point and had a new terminal clamp fitted for the battery. Again, since these actions the issues have all gone away.
Both the negative terminal being loose and the bonding point being corroded could cause the voltage being seen by the vehicle systems to be low or fluctuate depending on the load. This in turn could cause the instrument cluster microprocessor to glitch or reset and other systems to misbehave. We can't say whether all such faults can be fixed by such simple remedies but it's certainly worth checking as a first stage before looking at more complex diagnostics. Good luck!
[** End note: the instrument cluster removal / WD40'ing fix that has sorted some owners erratic instruments issue probably has a similar "root problem". Over a period of time the connector contact interface to the cluster may degrade, becoming more resistive and dropping the power supply volts to the instrument cluster by some small amount or affecting sensor signal paths. By disconnecting, lubing and re-connecting the harness, the contacts are cleaned and normal service may resume. Note that if the problem is with the harness connector you would be unlikely to also get the progression to the lighting flickering etc that I had]