When you say that it shakes when you stop at a red light, based on the context, I'm assuming that you mean when the car is fully stopped and the engine is idling. (If I'm mistaken about that and you mean that it shakes as it comes to a stop, I'd suspect a warped brake rotor.)
I'm going to guess that the engine is running poorly at idle after the engine is warmed up. This is a symptom for many kinds of problems, but three that come to mind are:
- vacuum leak - somewhere, if a vacuum line is cracked or hose disconnected, you will often get poor/rough idle as a result.
- MAF sensor problem - the Mass Air Flow (MAF) sensor measures the amount of air coming into the engine so that it can be mixed with the right amount of fuel. If the MAF sensor is not working correctly (for example, if it doesn't move freely due to dust, dirt or oil) the sensor readings won't be accurate and the engine will run poorly, including at idle.
- Cracked air plenum - if the air plenum is cracked between the MAF sensor and the engine, the symptoms will be similar to the above but for a different reason.
You haven't said if you have tried to use an OBD scanner tool on it. That would help diagnose this if you have or can borrow one.