Search never completes, but only with *some* terms

Question

Only for some terms that we have discovered (eg "backpack", etc., for other terms everything is fine) our Magento store search seems to time out after about 10s and returns a blank page saying:

"Service Unavailable The server is temporarily unable to service your request due to maintenance downtime or capacity problems. Please try again later. Additionally, a 503 Service Unavailable error was encountered while trying to use an ErrorDocument to handle the request."

What could explain such a selective and weird behavior and how could I avoid it?

I already tried modifying the configuration parameters in System > Configuration > Catalog > Catalog Search. I tried the 3 types of search (Like, Fulltext, Combine) and tested permitting layered navigation for any number of results, but nothing helped...

Answer 1

Debugging Magento, disabling extensions I found out that the problem was caused by an extension called "Indies CallForPrice" that was installed a long time ago, and unused. Once disabled the search started to work again for all terms.

I cannot explain the weird behavior though (why it worked for most terms and selectively did not for others).