1995 Honda Prelude radiator fluid bubbling

Symptoms: cooling system losing fluid and bubbling after hwy driving. Never overheated. Ever. No smell or white smoke from exhaust pipe. I change oil every 3k and I can't really tell if there is water in oil.
Had it pressure tested and found only that it needed new radiator cap.
Change thermostat? Water pump? Had it replaced about 30k ago. Should I simply try to replace thermostat? Should I simply add Steel Seal as it might be a leaky head gasket. This has been a great car but it does have 260k miles and, since it has other small issues, like cracked exhaust manifold, warped rotor, etc, and might be worth $3000 I cannot see dropping alot into it.
I drive about 35k miles per year so need a reliable car.