ford cooling problem?

56 reg focus 1.8 tdci
fitted new T belt and WP. engine runs fine, but takes ages to reach operating temp. water in header tank stable, hot air through blower, does not over heat. gauge tends to fluctuate alot, pointer will reach first marker then drop to zero before settling just below half way. im sure all the air has been bleed from system as the car has done over 1000 miles.