Calculator stops working after doing some calculations

Calculator sometimes stops working and does nothing after 4-5 operations . non of the keys work (nor make beep sound) and screen freezes . only recoverable with opening full reset menu (ON + RESET) which resets calculator and all calculations get erased . in BASIC mode , it stops after executing 5-6 commands . sometimes in CAL mode , keys beep when pressed but screen freezes completly . The calculator is not connected to anything and there's no ram card inserted.