Whenever the MySQL server experiences heavy load, Zope seems to hang, such that we can't even access the ZMI.
Is this a bug in version 2.7.6 or ZMySQLDA ?
Seems that all your Zope threads are busy (they're waiting for database response). There is no more threads to serve next requests so they're waiting (you may say that zope hangs in this situation, but in fact all it's threads are waiting for DB). You may install DeadlockDebugger product and you'll be able to see what your's Zope threads are really doing.
Or are there some settings I can tweak to adjust some database connection timeout value ?
AFAIK not in Zope. Take a look at ZMySQLDA or rather the python MySQL library it uses. Maybe there is something like that. You may ask this question on Zope-DB list. -- Maciej Wisniowski