Issue: Simple search in Fiction section slow and error

When using the search function in Fiction, within a category (using the "Search This Category) and entering key words in the box (1 or 2 keywords, using simple search), the search takes time and ends up with either a 500 or a 503 error message (see messages below)

Using the advanced search function does take less time and no error message

Test
- date & time: dec08 between 5:30 and 5:35pm GMT+8 (shall be after daily backup).
- browser: Google Chrome (last update)
- No other apparent slowdown perceived in other part of the forum