water & ice taste bad YUCK! PLZ HELP!

I have a Frigidaire [SIZE=2]GLHT188WHW[/SIZE]Top-Freezer Refrigerator with Water Through-the-Door Dispenser & ice maker. I have had it for about 3 years now. I change the water filter every 4-6 months, but have been needing to change it much more now (every 2-3 months). The water & ice have a dusty/musty/old taste to it. The water tastes fine after I flush out 2-3 pitchers of water, but that only lasts a day before I have the taste back. When I use the ice I can taste it after the ice melts some in my drink. Is there a system flush I should do, or replace something b/c it could have the black sludge (guess mildew like on sink faucet i clean off) somewhere in the waterline that I am not looking?