Another thought Johna, you could try warming the crystal up with a heat gun or hair dryer to see if it fails or the frequency changes, alternately get a can of freezer spray from an electronics store and freeze it to try and induce the fault.
It could be a dry solder joint, but my guess is a dodgy crystal. Heating and cooling may help in diagnosis.
Otherwise if any ICs are in sockets, remove them and plug them back in. Often sockets for IC's go high resistance with time, also pull any plugs out and reseat them as well.
Those are the easy things I can think of.
Cheerio
Pete