The pump has not been bolted down squarely. There is always a certain amount of movementin the bolt holes which means that when you screw in the bolts, you need to tighten them slightly so the pump can be moved but stay where it is when you move it. Once you're happy that the 2 gasket mating faces for the sump are at equal height, tighten the bolts down. There is the issue that because you are not fitting a new pump or gasket that you cannot do this without stripping the oil pump off and replacing the gasket.

HOWEVER,

It is very unlikely that this minor diference is causing the gasket to leak badly. If it has started to blow out the gaskets randomly, you need to check the breather and pipes are not blocked (the 2 ports off the rocker cover and one from the block to one of the rocker cover ports). You will also need to make sure the baffle in the rocker cover and the port in the block (there is a baffle in this port as well) are clear. These 3 points are what allows the pressure to escape so if they are blocked, the air can't escape.

Failing that then it is time to look at stripping the engine down as there may be an issue with work piston rings.