The easiest thing to do first is snake out the line in the event it's
clogged. If you still have problems, it could be a problem in the vent
line going up to the roof. It could be clogged and can be snaked from
the roof.
Another thing you can try to do is extend the drain pipe up about 2
feet. This will do 2 things:
1) Allow more water volume in the drain pipe, decreasing the chance of
overflow.
2) The pump should drain a tad slower because of the added height it
has to push the water up. This should not pose a problem to the
washing machine because most washers can push water up a max height of
8-10 feet.
Just make sure the drain hose is long enough for the added height.
Otherwise you can buy a drain hose extender.

It should be fairly easy to extend the pipe up. You mentioned it comes
out diagonally. Just buy a 45 or 30 degree elbow and a short length of
PVC pipe. Glue it together, then attach it to the existing pipe with a
no-hub coupling. This will make it easy to remove in the event it you
notice it still overflowing. ( If you have PVC coming out of the wall,
do not glue the extension otherwise you cannot remove it, just use the
no-hub coupling.)