Another thing to look into:
What does the oil pressure gauge look like when under way? Is it all over the place, or does it get up to about 30psi at idle and 55-60psi under load?
If it bounces all over the place, I recommend a new oil pressure sending unit. I know I was having a similar issue with the fuel pump power being cut on and off and it was due to the ECM "seeing" low oil pressure due to a faulty sending unit.