Engine Light and Traction Light On

[LexusRocking]

The engine light and traction control light went on simultaneously.  They stayed on for about 2 hours (I stopped and started a couple of times), then they went off.

I took it to a Toyota dealer (after the lights went off by themselves) who confirmed that the engine light and traction control lights had been triggered, but they couldn't diagnose a problem.  So, they suggested watching to see if it happened again.  The lights went on again yesterday, then off.  They are on again today.  

Any idea what's happening?