Q: Expansion tank cap keeps cracking and the car overheats in stop and go traffic.

The car is old and from the east coast so she's got a rusty undercarriage. Has had several problems but still runs nicely. Awhile ago it overheated for the first time. So bad that fluid sprayed everywhere. I took it in and a hose had come loose they reattached the hose. Immediately after leaving the shop she overheated again so I put water and coolant in the radiator. That worked for awhile but just kept overheating so I took it to another place. They did a bunch of work and replaced the radiator cap. Still overheated! I noticed the expansion tank cap was cracked so I bought the part myself and replaced it. Boom. No more overheating!!!! For about 6 months. Today, she overheated while I was sitting in bumper to bumper traffic. First thing I check? That freaking new cap was cracked in the same way as the old one. What is causing this? Some have told me there's coolant in the engine, or the thermometer is messed up. The fans don't seem to work (but they do sometimes). Help!! :)

My car has 180000 miles.