1983 BMW 320 Fuel pump problem

Fuel pump was whining loudly and making "slurping" sounds, car did not idle well particularly when cold: would rev up and down, killing sometimes when it went down. By adjusting idle to over 1000rpm - was able to control to a certain extent. Decided to replace system fuel pump and filter, also removed tank and checked in-tank "up take pump" and screen. Screen was almost completely clogged with rust dust. Cleaned screen and sent tanks (it's actually 2 split tanks) to be cleaned and coated at radiator shop. After replacing pump and filter - the problem seemed to be completely resolved (no noise, no idle problems) for about 150 miles. Now the new pump is whining and "slurping" and the idle problem is starting again but not quite as bad yet. First thought is to replace the in-tank fuel pump, but wanted an opinion before I spend the $250.