Check the feed is valid at
http://feedvalidator.org/ before you start looking for more complex fixes.
If all is well, one suspect would be overly conservative settings in your php.ini file which can cause longer processes to time out. Ask your server tech to modify your php.ini to:
max_execution_time = 300 ;
max_input_time = 120 ;
memory_limit = 24M
and see if those settings help...