I'm experiencing a similar problem, running the 2005-07-01 version. Access to feed.php from the browser is very slow and half the time I get the following error:
XML Parsing Error: xml processing instruction not at start of external entity
Line Number 2, Column 1:<?xml version="1.0" encoding="utf-8"?>
At other times it works.
However, trying to subscribe to a feed using a feedreader doesn't work at all and returns an XML parsing error. I've tried Thunderbird, Straw and Sage (Firefox plugin), all with the same (negative) results.