I've been having overheating problems, and have replaced the thermostat and checked the coolant, to no avail.
Here's the level of overheating: today was 95 degrees, and in stop/go highway traffic I was able to manage if I kept the AC off. If I turned it on, the needle would begin to rise. So I assume it's not a catastrophic issue like no fan or coolant.
Someone suggested that the fan wasn't kicking into high speed correctly - is this possible?

Not really...but if the fan CLUTCH was slipping, it could not spin up to speed as well, etc.

If the AC coming on is a cause, that increases the load, but slowly, not instantly.

It may be that you have too much anti-freeze in your coolant mixture, or, air in the cooling system, or, a loss of pressure from an air leak.

So a worn radiator cap, or, a loose bleed valve on top of the engine, etc, can prevent the cooling system from fully pressurizing.

Without pressure, it can't prevent boiling (like in a pressure cooker).

When it can boil, it can cavitate, and then bubbles are moving through the radiator, etc....and you can't exchange heat through the bubbled part of the flow, there's not enough surface contact to radiate the heat, etc.

If the needle is rising quickly, it might not be overheating though...it might be the temperature sensor that tells the dash gauge how hot you are.

Could also be that your radiator is no longer functioning well. How old is it? Are all the cooling fins still there? Are you in a salt environment?