I used Qt's threads rather than ruby threads and everything is lovely now. Maybe be something to do with the global interpreter lock.
I replaced
Thread.new { loop { every_minute_do_on_diff_thread; sleep 60 } }
connect(self, SIGNAL('every_minute_do_signal()'), self, SLOT('every_minute_do()'))
def every_minute_do_on_diff_thread
emit(every_minute_do_signal())
end
with
timer = Qt::Timer.new(self);
connect(timer, SIGNAL('timeout()'), self, SLOT('every_minute_do()'))
timer.start(60000)