Car fans are not allways working and overheat

Make sure your coolant levels are okay. If that is good, you may have a problem with your temperature sensor (which is located in or near the thermostat housing). Coolant also has to flow, if it isn't flowing it's hard for the temp sensor to do it's job. so you may have a problem with the thermostat, not opening when it should. A cracked radiator cap won't be able to pull in coolant from the reservoir. So a simple low cost way to proceed may be to inspect temp sensor (wiring harness/connector), remove and test the thermostat (replace if needed), replace rad cap (should be done every two years anyhow), flush and replace coolant. If that doesn't work replace temp sensor. There are guides on how to test the thermostat on the webs, which involves basically putting it in a pot of hot water and see if it opens.