chevy overheating

i have a 2001 chevy venture (3.4liter) it dont run hot sitting still but when i drive it starts moving over the half way mark then after about 2 more miles it bottoms out 'HOT' .WHEN I LOOKED UNDER THE VAN I SAW COOLANT LEAKING ON THE GROUND ITWAS COMING FROMSOMEWHERE UNDER THE THROTTLE BODY area but i cant tell where it's coming from . also i dont see the coolant leaking when it;s idleling , i only see it after ive drove it for about 10 miles could this coolant lost be causeing my overheating problem and do anyone have an ideal where the coolant is leaking from. i pressure tested it and the presure is good also i installed a new water pump and a new thermostat but i still have that leak and i dont know where it;s leaking from also i notice the coolant fan comes on at differant temps (dont it suppose to come on at the same temp all the time