96 3ltr v6 overheats only at idle in drive

My friend's 3 litre v6 '96 Taurus overheats but only when at rest while idling in drive. No problem when in motion or at rest idling in nuetral. This a right hand drive model in New Zealand. Drives fine and quite economical for it's age and engine size.

bleed the coolant system or check to see if water pump is getting weak.