Improving watercooling

[amie654]

My System spec:
Intel E8200
Geforce 8800GTS
Asus P5N-E SLI

Cooling:
Zalman Reserator kit + XSPC 120 radiator

Core temps (idle/load):
CPU - 57/70
GPU - 57/80C to 90C

Coolant temp
In 35.5
Out 36.5

What can be the cause of the large temperature difference between the cores and coolant? I use silver-based thermal grease, and the contact seems reasonable.