Hey mate,
ive had very similar missfire issues to you, using the same plug and yeh... 1-2 days and they get shitty..
I now use NGK BCPR6ES.. honestly... never looked back, car is perfect up until about 10,000kms and then replace when I notice a little bit of spluttering, but its still no where near as bad as those BKR6E's....
Also, u know, normal things... check all coil packs for cracks, make sure u have the right tension for the plugs etc etc...
hope this helps.
Adam