Sounds like you are filling it too full when cold. Water expands when heated. Therefore, it increases in volume greater than the capacity to contain it. The excess is forced out the overflow pipe.
Either let it establish its own level or drain it down to about half top tank. This should do it for you.
If that doesn't work then you may have other problems. Let us know if the above suggestion doesn't work and we'll try to analyze the problem and give you further advice.
It could be head gasket seepage, cracked block or head, or some flues in radiator stopped up. But given that it doesn't overheat, it seems that the simple may work.
Good luck,
Charlie