Last position in FMC frozen

[ARCO 245]

I am experiencing a very strange and new phenomenon with my FMC when I start a new flight. The POS INIT page remains stubbornly frozen on Gatwick geographical coordinates regardless of the actual location of the aircraft, meaning that when I enter the actual airport four letter code below with the new geographical data, I have such a variation that it generates an error message on my scratchpad.I tried this with many locations as varied as Newark, Geneva, Zurich, Frankfurt, Singapore, Manaus. No change, the "last pos" remains N51