I actually fixed mine!!!! Mine was acting up too. I found somewhere that the no/slow key response could be due to a short in the I/O socket. I blew into it really hard (you may want to use compressed air, but I didn't have any onhand), and it works perfectly now!!!!!!!!!! I hope this helps.

Here's how I fixed this problem. BTW, I had exactly the same problem: I could turn the calculator on and a black cursor would appear in the upper left but no keys would respond. I tried all solutions that TI suggested and others I read about, including upgrading the OS, resetting the calculator by removing all the batteries, etc. Nothing worked. Occasionally, if I started the calculator with the Del key depressed, it would display: "log(". But still no keys would respond.

I was about ready to give up but being a tinkerer, I decided I would take the calculator apart to see if anything obvious was wrong. I followed the instructions on disassembling the TI-84 here: http://www.instructables.com/id/Ti-84-Basic-Disassembly/

First time I disassembled it, I tried blowing on all the parts to remove dust. Then I reassembled it but the calculator still did not work. I then disassembled a second time and looked more closely at the main circuit board on the side against which the buttons are pressed. For each button, there is a gold colored star shaped pattern on the circuit board. I assume that the when a button is pushed, the pressure is detected by this electronic device. I looked closely at the log key because that was the function that appeared occasionally on the screen. I detected a very slight discoloration in the circuit board for this key. I want to emphasize extremely slight - in fact, my daughter, to whom I showed the circuit board, said she did not see it. I dunked a cotton swab into some rubbing alcohol and then rubbed the entire surface of the circuit board (side with the gold star shaped regions corresponding to the buttons). Then I rubbed the entire board with a dry end of a cotton swab. Then, I let the board dry for at least 30 minutes. I then reassembled the calculator and it worked like new.

My theory: the electronics that sense the depression of a key are very sensitive to the presence of dirt or dust. Somehow, some dirt or dust made its way under the keyboard and got lodged near the log key (it could have been any other key). It was a very minuscule amount of dust but enough to disrupt the electronics. The electronics then wrongly sensed that the log key was being continously depressed and with one key depressed, the calculator wasn't responding to any other keys, including the off key. When I cleaned the board with the rubbing alcohol, the offending dust was removed and the keyboard began to function normally again.

BTW: For those who are worried about taking the calculator apart, I say: what do you have to lose if the calculator is out of warranty? I am not an electronics genius by any means and I took the calculator apart in about 5 minutes. There are all of 12 screws to remove.