replaced fuel pump, but now it backfires an won't idle on 1990 Buick Skylark

it ran fine for about 5 to 10 minutes then started this problem, also replaced the fuel pressure regulator, but same thing, it did help.
why is it doing this, any ideas?

Asked by for the 1990 Buick Skylark
Did this ever get resolved, and if so, what was the problem? Thanks!