Oil Problem 1997 Disc.

I seem to be loosing oil presure after drive for 15 minutes, then the
engine looses power, and starts sputtering. I havent taken it to the
dealer yet, but my mechanic says he's not sure what the problem is, it
doesnt seem to be the oil pump, and we changed to oil and it runs fine,
then it starts the same problem again.

any suggestions, as to what may be causing this?

Thanks
Rich