I would suspect that you have either a Grey or Green cooling system pressure cap, and you should IMMEDIATELY replace this cap with a Black one. The Grey/Green caps run about 150 KiloPascals of pressure (22 psi), and this is significantly higher than what you need to have. A Black cap is only 75 KPA (about 11 psi) and much more suited to the realistic needs of the system without causing a burst hose or radiator. Many heavy duty SUVs and trucks use 20+ psi pressue systems, and your Volvo will function just fine at 11 psi, which is quite common for most passenger vehicles. Replacement pressure caps are available through your Volvo dealer, salvage yard, or favorite import vehicle parts source. (like FCP Groton for only $4)

If you already have a Black cap fitted to your car and you still have abnormally high pressures, try swapping it for another black cap. It is not uncommon for the valves in the pressure caps to lock up due to dried antifreeze in the valve assemblies. This can be cleaned out if you carefully dismantle the double valve assembly, but is usually easier and safer to replace the cap.

Also, carefully inspect the coolant reservoir for small hairline cracks. I recently worked on an '89 740 Turbo that developed a hairline crack in the coolant tank, lost pressure and coolant, and overheated and warped the head. The coolant tank is only $25 or so from the dealer (or FCP) if you need a new one, or a salvage yard replacement is usually acceptable.

I've also thought it was odd that the temperature gauge never went above the middle with any of these incidents.

You will probably never see your temperature gauge move past the 12 o'clock mark for the entire time that you own your car. The 700 series Volvos have a limiter circuit built into the gauge that prevents it from passing the middle of the gauge unless it's REALLY REALLY hot and overheating. Volvo's reasoning behind this was to eliminate normal fluctuations and aleviate the concerns of paranoid owners who saw the gauge moving up and down a bit as the thermostat opened and closed during normal operation. The 240s also have a delay/averaging circuit added for the '86-'93 cars, but this is easily bypassed. The 700 series compensation circuit is not so easily defeated.

The mechanic has discouraged me from any more repairs on this car because it has 310K on it - but this would be relatively inexpensive.

I would advise you to find a new mechanic. Your current mechanic probably spends most of his time working on Ford/GM/Chrysler vehicles that are considered "High Mileage" after 100,000 or 150,000 miles. However, there are NUMEROUS Volvos out there that are well past the 1/2 million and 1 million mile markers, and Irv Gordon has well over 2 million miles on his. There's no reason that your car can't go this long, but it requires keeping a regular service schedule as outlined in your owners manual (and a few other tuneup items that are common for cars of this age, keep reading this forum for examples). I would consider my '87 240 to be of "middle aged" now that it's past 250,000 miles, and I might consider it to be "high mileage" after it's passed 500,000 or 1,000,000 miles. However, any Volvo can be brought to an early end by a neglectful owner. If you're missing your owners manual or service schedule (also called a Maintenance Chart), you can download a replacement one from the Volvo USA online Library.

how likely is it to be the head gasket?

I would suspect your head gasket is fine, unless the engine has been overheated. A quick and simple cylinder compression test will determine if your head gasket is good. You can pay your mechanic $50 to do a compression test, or you can buy a compression tester for about $35 and perform the test yourself. The instructions are in the Brickboard FAQ under the Compression Test section. A blown head gasket should not burst a coolant hose, the excess cooling system pressure should be relieved through the pressure cap (assuming the pressure cap is venting pressure properly and is not over-rated for the system).

FYI, here are the commonly found Volvo pressure cap colors and their equivalent pressures:
Grey: 150 KPA, 22 psi.
Green: 150 KPA, 22 psi.
White: 100 KPA, 14.5 psi.
Black: 75 KPA, 11 psi.

God bless,
Fitz Fitzgerald.
--
'87 Blue 240 Wagon, 254k miles.
'88 Black 780, PRV-6, 146k miles.