highway vibration

My '99 SW1 with 140,000 miles, has started to vibrate around highway speeds. Not just 1 speed, and not all the time, and sometimes worse than other times. The tyres are balanced. The top motor mount has been changed about 2-3 years ago to cure a previous vibration. I've read it may be the transmission side motor mount. How do I check this? Any other thoughts are welcome too.