Check wiring at ECU and between main fuel relay & fuel pump for shorts and loose/dirty connections. (note: The ground connections I mentioned before are very important. Bad grounds can cause all kinds of weird intermittent problems.)

What code(s), if any, does the ECU show when the problem occurs?

When was the last time you reset the ECU?
(by pulling the ECU fuse for a few minutes; Sometimes, it can get strange data stuck in its memory that can cause problems.)

Take Care,
ChrisS