Anti theft preventing my car from starting

Take the key to the dealership and have it tested. It is possible that there is a problem with the chip in the key head and it is causing your issues. If so, all you need is a new key. Try that and let us know what you find.

When you put the key in the ignition and turn it to start and run, what does the security light do? It can give you an idea if it is the security system by what it does.

That would mean the system is not recognizing the key you have. However, you say that it sometimes does start? Does the light stop flashing then or does it go on solid?

It is not the key itself that is the problem. It sounds like the pass-lock sensor or its wiring has a problem. What do you have on the key ring along with the key? Anything?

Your car has the early pass-lock system, there is a magnet that turns with the ignition cylinder, this passes a sensor inside the ignition switch housing. That is the part that triggers the alarm on/off. If it does not get the signal for more than five seconds the light flashes and the car will not start. This is a sort of common issue. The repair is usually a replacement sensor inside the ignition switch assembly.

Unfortunately the only real way to tell if it is a failed sensor requires a scan tool that can watch the security system as it responds when the key is moved. Either a good shop or a dealer would have those. Then once the sensor or the wiring is replaced there is a ten minute relearn process using the same scan tool.