I have a 2002 mitsubishi montero sport and we changed the thermostat, did a radiator flush and change radiator cap and the needle passes to mid point (normal) it hasnt overheated but it doesnt stay in normal. when i do not turn on the ac it stays normal. what can the problem be and how can we fix it.

Where do you live?
This has been a hot summer, bit hotter than usual due to the solar flares going on...
as long as the pointer stays within normal operating range, you must not worry... especially since you changed the thermostat and did a flush, did you make sure you got the right temperature range thermostat installed, rarely but few times that seems to be the problem, hotter thermostat gets installed by mistake.