Can not get a reading on the diagnostic machine. Why?

I have a Lexus SC400 1993. I have a diagnostic and when I plug it in to the system it say cannot read. I bought a tester light and it has power going to it. Why will it not read the diagnostic machine? ALso it will start, but it acts like it want to die out and the RPM are going up with out putting my foot on the gas peddle. After the car sits for a while it will start and run until it reached a certain temputure and then goes blog. The engine light also on. Can you help me?