MAF sensors on MY99 are notorious for failing. In my early days of Subaru ownership I lost two engines to this. On an MY99 UK Turbo 2000 which was a watered down WRX.
You're showing classic signs of a failing
maf.
As others have said reuse the standard sock from your OEM pump. Don't forget the little clip that holds it to the fuel pump body and press it inplace with a small flathead screw driver. They tend to bend and not clamp once you remove them once. The DW300 may be a little different but I've just done mine on a Walbro 460 so its pretty fresh in my mind what needs doing.