i had an extremely similar problem to ures. only diff is mine is a rb20det in an s13 chassis. i went for a whole year changing my coils packs, spark plugs, fuel pumps, timing etc...
my problem turned out being when my engine was installed there were 2 identical plugs for the knock sensors. they plugged in a plug that went to the fuse box, so there was no reading from the second knock sensor causing the ECU to think something was up and drop power. i had a look and found a plug that went to the ecu, plugged that in and hey presto all is good.
moral of that story is check your knock sensors and that the plugs run to the right palces i.e. ECU, might save you a lot of money and headaches...