archive.php errors are a mystery

We're running piwik tracking 700+ sites with something like 200-300,000 hits/day, evaluating it alongside another analytics software to decide which we will use for the long run. We're running archive.php from cron every hour, and for the most part it runs without error.

But 2 or 3 times a day archive.php will have an error like this:

Error: Got invalid response from API request: http://mydomain/index.php?module=API&method=VisitsSummary.getVisits&idSite=778&period=day&date=last2&format=php&token_auth=token&trigger=archivephp. Response was '<br /> <b>Fatal error</b>:  Exception thrown without a stack frame in <b>Unknown</b> on line <b>0</b><br />'
[2013-04-26 10:15:50] [17123131] 1 total errors during this script execution, please investigate and try and fix these errors

I have browser archiving turned off and reports processing set to 3600 in Settings. A few things I've noticed:

*archive.php sometimes has date=last2 and sometimes date=last52
*The error, when it happens, always happens for the first 1 or 2 sites, which in our case is 778 and 779.
*Looking at 'ps', I see sleeping archive.php processes from long ago which never finished/exited. For example:

root      2226  0.0  0.0   2516   884 ?        Ss   00:05   0:00 /bin/sh -c /usr/bin/php /var/www/html/piwik/misc/cron/archive.php --url=http://mydomain/ >> /var/log/piwik-archive.log
root      2230  0.0  0.3  32592 14688 ?        S    00:05   0:00 /usr/bin/php /var/www/html/piwik/misc/cron/archive.php --url=http://mydomain/
root     17524  0.0  0.0   2516   880 ?        Ss   Apr25   0:00 /bin/sh -c /usr/bin/php /var/www/html/piwik/misc/cron/archive.php --url=http://mydomain/ >> /var/log/piwik-archive.log
root     17525  0.0  0.3  32592 14688 ?        S    Apr25   0:00 /usr/bin/php /var/www/html/piwik/misc/cron/archive.php --url=http://mydomain/
root     19478  0.0  0.0   2516   884 ?        Ss   01:05   0:00 /bin/sh -c /usr/bin/php /var/www/html/piwik/misc/cron/archive.php --url=http://mydomain/ >> /var/log/piwik-archive.log
root     19483  0.0  0.3  32928 15044 ?        S    01:05   0:00 /usr/bin/php /var/www/html/piwik/misc/cron/archive.php --url=http://mydomain/
root     32245  0.0  0.0   2516   888 ?        Ss   Apr25   0:00 /bin/sh -c /usr/bin/php /var/www/html/piwik/misc/cron/archive.php --url=http://mydomain/ >> /var/log/piwik-archive.log
root     32246  0.0  0.3  32592 14780 ?        S    Apr25   0:00 /usr/bin/php /var/www/html/piwik/misc/cron/archive.php --url=http://mydomain/

Can anyone offer an idea about what may be happening here and why we get these intermittent errors? My first thought was that maybe the first site(s) sometimes takes a long time to archive. But the archive.php log shows that less than a second goes by:

[2013-04-26 10:09:03] [17123131] ---------------------------
[2013-04-26 10:09:03] [17123131] START[2013-04-26 10:09:03] [17123131] Starting Piwik reports archiving... 
[2013-04-26 10:09:03] [17123131] ERROR: Got invalid response from API request: http://mydomain/index.php?module=API&method=VisitsSummary.getVisits&idSite=778&period=day&date=last2&format=php&token_auth=token&trigger=archivephp. Response was '<br /> <b>Fatal error</b>:  Exception thrown without a stack frame in <b>Unknown</b> on line <b>0</b><br />'
[2013-04-26 10:09:03] [17123131] WARNING: Empty or invalid response '' for website id 778, Time elapsed: 0.123s, skipping
[2013-04-26 10:09:07] [17123131] Archived website id = 779, period = day, Time elapsed: 4.127s
[2013-04-26 10:09:10] [17123131] Archived website id = 779, period = week, 1860 visits, Time elapsed: 3.351s

So the mysteries are:

(1) Why the intermittent errors on the first site
(2) Why are some archive.php processes not exiting but sleeping forever
(3) Why is date sometimes last2 and sometimes last52
(4) Most importantly, are these errors and sleeping archive.php processes causing data to be lost for the affected sites?