I've experienced the same problem. I work on 300-450 page manuscripts where FIND is essential. The size of my files may be part of the problem as I don't experience FIND quitting on me on files of a few pages or less; I also don't work on them for near as long and collect as many saves, etc..
I talked with a senior advisor from Creative Products (or whatever they call it) at Apple and he confirmed they know about the issue, but haven't fixed it. They are still trying to collect data (logs and screen actions) from users experiencing this problem. Call APPLECARE when FIND quits and have them collect your non-working data logs, etc. for their Pages engineers. Might help accelerate a fix.
My current workaround has been to DUPLICATE my file where FIND/FIND & REPLACE are not working and throw away the non-working file version. It's not a permanent solution but it works for a while until it doesn't, then I do it again.