I had this issue once on my Beta - no problems at all until oil and filter change with an after market filter. Changing the filter for the OE version cured it and haven't had the problem since. It could be that the cross reference Fram - Mahle isn't correct and you're already one 'generation' removed with the Isuzu OE - Fram conversion....
The internals may be completely different - flow 'inside to outside', 'outside to inside', 'plugging' valve, no 'plugging' valve, the permutations are almost endless....
I'd try an OE filter first.