Like Russell says you are not really going to get any improvements by messing with the crystal and if you are seeing huge errors then its almost certainly not the crystal thats causing them.The standard way to get very high crystal stability is to use a temperature controlled oven,but you really souldnt need to do anything like that at all if your software is working correctly.
When you simulate you only simulate a few minutes run time at most,well I do anyway,rather than hours which errors build up in.