I have an old wooden cased mantel clock (probably at least 50-60 years
old). It's been running fine since I got it in 1992.

It's now begun to stop intermittently. Always at about 5 minutes to 11.

I've checked that the hands aren't fouling each other - they are not.

Any tips what to look for before I find a clock repairer?

--
Use the BIG mirror service in the UK:
http://www.mirrorservice.org

*lightning protection* - a w_tom conductor