Found specs on TA 7358AP; rated from -25 to +75*C . Obviously it is
not the problem. The PVC1 and PVC2 on the other hand are most likely to
cause the terrible drift,especially now, in the hot summer days. Their
internal structure contains polystyrene dielectric,as the tmperature
rises plastics expand as far as I know. This will inrease the
capacitance very minutely, but apparently enough to be seen on the
digital display . One way to tackle this is to add a component with
opposite thermal rating- capacitor or inductor in the oscillator/mixer
circuit.