I've got a '98 Renault Laguna 2L RT Sport. Every now and then, usually
on a longer, rather than shorter run (15 miles plus) the coolant
temperature gauge, which normally sits exactly on about 55%, suddenly,
in the space of 30 seconds, cranks up to 98%, at which point the a/c
cuts out. From then on, it will creep down to about 95%, at which point
the a/c cuts in again, then within a minute creep up to 98%, a/c cuts
out again, then down to 95%, and so it continues. However, if I pull
over, turn the engine off and immediately start up again, it's
immediately normal for the rest of the journey, as if it's been reset.
First I replaced the gauge temperature sensor and the thermostat, to no
avail.  Since then it's been to the local Renault dealer, a cooling
system specialist, and an auto electrician spent an afternoon working
on it, but no-one can find the fault. I now know for sure the cooling
system is fine, it's not overheating, the high temp. warning light has
never come on, engine diagnostics confirm that the coolant temp. stays
at 91 deg. whatever the gauge says, it's never boiled over or used any
coolant whatsoever, and this fault has endured for two years now! It
almost looks like a software glitch in the engine management system.
Has anyone had the same problem? Any solutions? Email me please at
johnwallis75@hotmail.com