OVERHEATING?!?!?!?

when was the last time your fan clutch was replaced? coolant leaking from your water pump or thermostat housing?

You need a real gauge in order to do any diagnostics.

What did you check so far?
First thing i'd do is check the coolant level.
How old is the thermostat?
Condition of the radiator?
Doubtful it's the water pump, usually they are either good or bad but no in between.

Hissing is right, the temp gauge isn't all that accurate, but usually is effective to know if you are near overheating or not.

Overheating

If you mean that idling or in traffic your temp. gauge is reading high then goes to normal when at road speeds, then you could have a problem. Check what the others mentioned.
Coolant Level
Fan Clutch
Clogged Radiator
Two items I would add is the radiator cap. Make sure it seals.
Antifreeze is near 50/50%. It gives max. cooling effect at 50/50.
It could be with engine modifications you have exceeded the cooling capacity of the stock radiator.

overheating or getting hot at low speeds usually indicates the fan clutch isn't working or if it's an electric fan it's too small, at higher speeds usually the radiator is the problem, more of a get you in a direction to check things than a rule of thumb. The others gave great advice..1st the stock gauge isn't a true indicator of anything get an aftermarket gauge to get a better reading on the temp. EFI cars run at 210* normal temp. do all the other stuff already stated to check things, also check to make sure the air defelector is still on the car, it's mounted under the core support and directs air up into the rad..these cars are bottom breathers. A pressure test also may be needed and a check to make sure there is no leaves etc. trapped between the a/c condensor( if you have air) and the rad to block air flow. you haven't maxed the cooling system with your mods.