Please see note from Sullivan, in this thread:
http://www.brickboard.com/RWD/index.htm?id=1034570

Normal failure of in-tank pickup hose:
Runs poorly when low, runs well when full.

Normal failure of in-tank pump:
Runs poorly when low, as with bad pickup hose

Normal failure of main (external) pump:
Car does not run. Period.

Clogged fuel filter:
Runs poorly when demanding lots of fuel (acceleration, hills)
Volvo fuel filters last for many tens of thousands of miles, maybe more than 100K?
--
Sven: '89 245, IPD sways, electric rad. fan conversion, 28+ mpg - auto tranny. 850 mi/week commute. '89 245 #2 (wifemobile). '90 244 (spare, runs).