RSS import problem – incorrect publish date

I have a large XML formatted text document that’s used to import posts. Everything works fine if I RSS import 59 posts at a time. However when I try to do import 60 or more, the publish date reverts to 1/1/1970 for a certain portion of the items. This is a problem because the publish date has been set to consecutively order the posts. I’ve tried this on two local WP and one remote WP (all v.2.9.2) and get the same results. Here is an example of my XML file:

<item>
 <pubDate>Wed, 30 Jan 2009 12:00:00 +0101</pubDate>
 <category>Searchable</category>
 <title>Title one</title>
 <content:encoded><p>New paragraph number one</p>Page 2</content:encoded>
</item>
<item>
 <pubDate>Wed, 30 Jan 2009 12:00:00 +0102</pubDate>
 <category>Searchable</category>
 <title>Title two</title>
 <content:encoded><p>New paragraph number two</p>Page 2</content:encoded>
</item>
<item>
 <pubDate>Wed, 30 Jan 2009 12:00:00 +0103</pubDate>
 <category>Searchable</category>
 <title>Title three</title>
 <content:encoded><p>New paragraph number three</p>Page 2</content:encoded>
</item>