i just visited my friend's workshop who is having a sigma vmc with a fanuc on it. He told me that the machine was dropping the tool in the middle of the cutting operation (as you said it happened randomly, sometimes with the first tool sometimes with the second) The reason came out to be was humidity and moisture in the atmosphere that was giving a fake signal to it.
So he just added a push button to the control that he use just when the machine go for a tool change. rest all the time, the controller was unable to tell the plc to drop or change the tool.
i am not sure if your issue is similar to it but you can think this way too (specially, if your control and drive panel does not has a cooling system with moisture control)
my friend's sigma vmc has just some air filters and use external air for cooling.