Bigwulscoobydoo

Hi there first of all great site guys
Im new to this subaru buisness as i was used to driving the old German tanks (BMW's).

Just got my self a 1998 r refg subaru impreza catalunya no. 199/200
It has the bug eye shape subaru Engine now in her,

What the problem seems to be is
When im driving along the road the heaters blow nice hot air as should be...
Its then when im sittin in traffic of sitting idling the car water temp gauge rockets up and the heaters start blowing cooler air altho not cold if this happens i give a rev of the engine or drive on again the temp gauge returns to the middle position and the heaters are hot again????

Its a strange one and im worried it may be signs of the head gasket failing??
Any help would be much appreciated

cheers

wullie.