I have it. it seems to be very common on the 88-89 Z31s. no one knows why it does that. Someone have said they would try replacing the coolant temp sensor but no one ever reported back to let us know if it worked.
CTS is in the passenger head, between the first and second fuel injectors. Easy to see and easy to remove. I have the same problem, and it continues even though I've replaced the sensor, though (see thread below). If that happens to you after replacement, it seems the best advice you'll get afterwards is to "smack the console a couple of times", so I hope the replacement works!