Pressurize the cooling system to raise the boiling boint and look for leaks. Could be a pin size hole dripping (normally at idle or after turning the car off)and then the faster you go could be sraying a tad towards the motor witch would hit the header and smoke. Check the upper and lower radiator hose for leaks.

93 montero sr 3 inch body lift 31x11.50 super swamper ltb, 2 8inch 130 watt pro comp off road lights, 2 55w lights on the rear bumper, rear air locker, magnaflow exhaust muffler,superwinch manual hubs,15x8 black rock crawlers (steel)