I would be very surprised if your cluster repair proves to be a long term fix for the speedo zeroing problem. The Focus suffers from two faults in this area. One, as you have found, is poor solder joints on the instrument cluster, and the second is an internal fault in the VSS (Vehicle speed sensor). This item is in the top of the transmission housing and supplies the ECU with data relating to vehicle speed and displays this on the speedo and also has an input to fuelling. Once replaced with a new updated item, it is unlikely you will suffer this problem again. A difficult item to change, and costs around £40, but certainly worth doing.
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