Mine did exactly the same thing. Replaced with a new square Facet electric fuel pump which helped a little. Changed to Fuelmiser which increased flow by heaps but not pressure (did the flow/timing in a 1 litre container test) and never had a problem again, even at 90mph.
Can you provide the model number of Fuelmiser pump you used? Also, did you need to install a fuel regulator with it? Thanks.