Coolant/Overheating problem.HELP!!

[DFLROWLAND]

I have experienced some problems with the coolant system which is confusing me as to what is exactly wrong.  
 
Basically the car was left running for approx 20 mins and then temperature guage went right up and the coolant light came on.  The car was switched off straight away.  There appeared to be air in the coolant system.
 
The next day  I topped the system up and the car appeared to run fine and after about 10 mins driving the coolant light came on again and the temp guage went up.  30 seconds later the temp guage dropped back down and the light went out.  This happened for the rest of the journey with the guage going up and down.  However, the greddy gauge remains steady reading between 75-90C.
 
This leads me to believe it could be a fault with the instrument cluster guage, air in the coolant system or a faulty pressure cap.
 
I contacted Rotechnics and they didn\'t like to commit over the phone but said that the pressure cap is a common problem.
 
Does anyone have any ideas based on there own experiences as to what the trouble might be?

Any help would be greatly appreciated.

Cheers.

[Andy_k]

Could just need the air bleeding out of it.

Can you see white smoke (from the exhaust)?
Or any steam or water on the floor or engine bay?

Glad you got it in the end, lets just hope its an easy problem.