hello i have a sierra cosworth saff

i just fitted a rs500 intercooler so i hand to take out the radiator,
put it all in and filled up with coolant and water.
now the temp gauage is reading abit below when it used to when i am driving it,the needle sits just below the N on normal, but as soon as you stop moving and get in to town with slow moving traffic and traffic lights the needle drops right down to the bottom of the white square of the temp gauage,
the i went though town and back out on to roads at 60mph the gauge just came up alittle??

i pulled over and checked all the pipes and they were warm and could see the water returning to the header tank???

can out the next day and felt the pipes when they were cold and they defo had water in as could feel it in them. the header tank was on max ??

help me please

cheers gareth