Yes...Signs of overpressure

And yes you're using a good coolant (Zerex G-05) adding more safety against overheating.

Though I'd recommend using Volvo's latest coolant G64 Glysantin (part# 31439720) designed to more effectively remove engine heat. Yes its compatible with older Volvos 700 series included.

My 940 now using G64 takes about 1.5 minutes of radiator fan activation to cool down, compared to about 2 minutes when using old OE coolant.

With the headgasket recently replaced (still in its infancy) I tend to think of something often overlooked by many owners

Consider checking for half-failed thermostat, radiator fan clutch (if equipped) or electric fan (if equipped).

The fan clutch could faulty or the electric fan could be more than half worn (ie. still spinning but not at full power)

Replace the grey cap (if long not done) while you're at it.

Amarin.