Brightness changes too frequently after I updated to 4.2
Unfortunately it is a known bug introduced on Nexus 7 with Jelly Bean 4.2. In previous version 4.1.2 auto-brightness was criticised as slow in reaction on light changes and now in 4.2 it is just opposite, over-reactive. From one bad to another bad. But I agree with you that even previous not so good behaviour was better than what we have now. Someone cure it by sticking small semi transparent tape over light sensor. But it is only temporary and ugly workaround, we can hope for correction in the next version, or downgrade to previous 4.1.2 version.