I had a similar issue with my Nexus 7. Would only boot normally if I turned it off with the charger in. (Also, it would only show the charging icon while powered off if I plugged in the charger before turning it off.) Otherwise, I had to boot by holding the power button for 20-30 seconds. I saw this post and removed AVG Antivirus to see if it helped, but it didn't. (I didn't wait 8 hours like you did though...) I contacted Asus and they suggested I do a factory reset, so I did, and so far so good! Haven't tried reinstalling AVG, but I may just to see if that was the issue. Don't really like having to factory reset all the time though. Have you found anything out since you removed the most recent antivirus program?