Slow Search all of a sudden
 

I run a rather large forum (Threads: 146,633, Posts: 3,588,904, Members: 31,026) and about 2 weeks ago my search started getting very slow, it takes about 10-15 seconds to get results. First thing I did was repair/optimize my search table and this fixed the problem... for a while. At the end of the day, search was back to crawling again. I've been repairing/optimizing the table every day but after a couple of hours it slows down again...

Any idea what could be causing this?

Server load is fine...