FreeRTOSAT91SAM7X UART Overrun problem

Hi all, I have a problem concerning the SAM7X USART exploitation using FreeRTOS. Such USART is connected to a GSM Modem. The read-write operation are under interrupt handler. When the USART interrupt is enabled, the firmware handler checks the cause through status register:
• In case of new byte to read, the byte is extracted from US_RHR register and sent to a queue using xQueueSendFromISR. In the meantime, such queue is processed from uartReceive function;
• The writing phase follows the opposite process. From UARTWrite function, a single byte is sent to queue and the interrupt is required. Then the bytes is extracted from queue using xQueueReceiveFromISR and is written to US_THR register. The problem is that I detect some Overrun Error (overwrite operation of the US_RHR register). Obviously, the faster is the baudrate, the worse is the problem. Till now I was not able to perform some specific test with oscilloscope in order to evaluate the time duration of the interrupt, but I guess that the problem should not be the usart handler time occupation. Could you please give me some useful advice on that? In your opinion the hardware handshaking should solve the problem? Should I change strategy (for example using DMA?) Many thanks in advance. It should be very important to me to solve it. Best Regards, Antonio Defina