The air conditioning cutout is a common problem on this
model. The Klima relay implements a function which
compares the a/c compressor RPM to the engine RPM. If
the two differ by more than a small margin, the a/c is
cutout until the ignition is cycled. This is what is
occuring to your car.

There are several possible causes, so don't be to
quick to replace the Klima relay. Other possible causes are:
1) Compressor clutch gap. The gap is specified as 0.5mm
+- 0.15mm. As the compressor ages, the gap increases.
By the time it reaches 0.8 or 0.9mm, compressor
engagement becomes quite erratic.
2) Grease/oil on the compressor clutch. Sometimes a simple
degreasing can cure the cutout problem.
3) Corrosion on wiring. Any problems in the wiring which
carries the compressor speed signal back to the Klima
relay can trigger the cutout function.
4) Not certain on this one, but I believe problems in
the engine speed feed to the Klima relay can also
cause problems.
5) Worn serpentine belt.
6) Defective belt tensioner.
7) Missing plastic splash shield. The plastic tray below
the engine prevents water from getting on the serpentine
belt. Water can cause enough slipping to trigger the
cutout function.
8) Defective Klima relay

The other relay your mechanic is referring to is the
overvoltage protection relay. These are known to fail
with age. However, I don't think it affects the operation
of the tachometer. Rather, I'd look for corroded connections
in the path from the engine speed sensor to the tach. If
the engine computer (EDS) cannot see the engine speed, it
will not attempt to regulate the idle speed, resulting in
the low idle speed. A failing OVP relay can also shut
down the EDS, so it could be the culprit. The OVP is not
too expensive (about $35, I think) so it may be worth
replacing.