CC2640: CC2640 BLE stops working after a while, with UART function enabled

Part Number: CC2640

Chipset : CC2640

BLE Stack version:  ble_sdk_2_02_00_31

Problem description:  

Using CC2640 as an observer, based on simple_observer code. The chip scans for BLE devices, collects broadcast data and then sends out to PC through UART. The system works well at first couple hours after powered up. Then the chip gets into a non-responsible status. No data is sent from CC2640 to PC through UART but commands can be sent to chip through UART. Based on debugging logs, seems the chip can not scan for devices and collects data any more. Does anyone run into this kind of problem ? BTE, the "couple hours" window varies during several tests.

Searched online, seems has something to do with memory leak caused by BLE stack. Does any one have similar experience?

Any help will be greatly appreciated!