tj1 is right - this is likely a problem with your webhost limiting processes to 30 seconds, but the task you're attempting to do is taking longer than just 30 seconds to process.
You could try this workaround, which will hopefully allow it 5 minutes to process instead.
edit /admin/store_manager.php
around line 25 you'll see this:add this above it:
Code:
zen_set_time_limit(600);
.... which makes it look like this:
Code:
zen_set_time_limit(600);
switch($action) {
NOTE: Some hosts don't allow the overriding of their 30-second-max, in which case the above change will have no effect, good or bad.