obars

I have a 1979 Corolla TE51, SR5 with 1600 motor. The mechanical fuel pump is going bad. I decided that rather than twist myself into a pretzel and replace it, I would install an electric pump instead. Well I did, under the hood, low on fender well. Worked for about 8 miles, then started cavitating even with full tank. So I moved it just in front of the tank, lower than tank, put a fuel filter before it. I worked for about 5 miles and did same thing. Anyone have any idea whats going on. If i hook the old mechanical pump back up it runs great, just has a seap of fuel leak on it. I tryed also, to plug the return line from the old pump, when electric was installed, and same problem. Thanks for any help. John