1998 Honda Accord Drop In The RPM

The Car's idling is set to around 1000 rpm, it usually runs normally but sometimes when I have driven constantly and start the car, the rpm drops to 200-250 rpm and acts as if its almost dying. This thing stops only when I turn off the engine and wait for the car to cool. When the drop happens the exhaust sounds different like its blocked.I dont know which part should be replaced or serviced. Please advice on what should be changed