Overheating

Ok i have a 97 mazda 626 and its been overheating.. problem is, it never *really* overheats. The temperature guage is always floating towards hot, it'll go up then come back down again. Or sometimes it's all the way hot, but the car just keeps going like nothing's happening. I took it to the mechanic and he said he couldn't find a problem.. he changed the coolant and did a couple other things but I'm still having the same problem.

Is it possible the guage is just broken?? Or do you think maybe the radiator is a problem? I had it in the shop about a year ago and the guy said it was cracked.. he said eventually it might become a problem. I dont know but the whole thing is weird as hell, I've never seen a car act like this

any help is appreciated. thx

Some things to check:Radiator cap may be bad and not allowing car to operate with proper coolant pressure,water pump is bad or going bad,thermostat is bad and needs replacing,overflow coolant bottle has crack in it and is allowing air in the system.Beyond these simple checks, it gets bad like blown head gasket. BJ

INdeed...these episodes look like the cooling system is filled with puffs of air/exhaust. IF this is the case, so often contemplated "vacuum purge" of the cooling system will lead nowhere. Do you get normal continuous heat from the heater (when you set it on heat, of course). Blown gasket invade the cooling system with exhaust gases and this makes the heater provide heat for some periods then provide no heat at all. If you have such a symptom, it is definitely a blown head gasket. Not having this symptom does not clear that option though.