If you have cleaned the fan...the only other thought would be the fan is failing. Use a fan speed monitoring program to watch the fan speed and the cpu temperature. Also, when cleaning the fan make sure not to spin it fast using the can of air. That will shorten the life of the fan.
The X60 is usually a rock solid performer. I had nothing but good luck with mine.
Ya, I love the X series
I've never had one fail. I've passed on most of my X machines onto family members that just need a computer for email / web etc. A X30 with a 7200rpm HD isn't too bad even compared to today's horsepower. Running rock-solid to-date.
I'm suspecting this "overheating" more likely a Windows 7 induced issue, maybe coupled with Lenovo's Window 7 software.
I notice in the Lenovo Power Manager (I'm running 3.45) that it's optimizing performance based on establishing "passive" as opposed to "active" cooling. Supposed to reduce power consumption (by reducing cpu usage etc). I may try meddling with this.
I have a spare X60 that I bought for cheap off ebay last year that is laying in wait. If something ever goes on this primary X60 (that I've owned for 4 years, I use for work and play, so 12+ hour a day, 365 days a year, for 4+ years), so this X60 has really live a great life. The bezel on both sides are breaking away etc (lost the pcmcia door, etc).
I have X2*s plugged in around the house for various purposes, and they too are rock-solid.