1995,Leaking radiator cap

I noticed my car engine smoking from the front, when I checked I saw that coolant had been leaking from the cap then down the back face of the radiator causing the smoke I'm assuming. My overflow tank had been drained down to the minimum line so I refilled it and a few days later it was empty again. My car only starts smoking when the overflow is filled and stops when its near empty. My temperature gauge on my dashboard has not gone past half way where it always goes up to so I;m assuming my car is not over heating. I just want to know why that is happening and what to do to fix it.

M