99 intrepid overheating......PLEASE HELP

Just recently bought a 99 intrepid from a private seller. Drove it home back to store and home again. Fine. on way to work overheated. On way home Fine. Went to DMV this afternoon fine. after that went to find about about an allignment it overheated. Drove home. nothing it was fine.
The previous owner says radiator and thermostate was just replaced.. what else could be the cause?? Thanks a bunch!!! Mary

coolant level low, air in the system, faulty waterpump...