Communicating dsPIC with PC application through UART. Receiver interrupt handling

Question

I'm communicating my C# applications with dsPIC x16 microcontroller using UART. I want to send/receive fixed size frames and I tried to manage it in a following way:

if(readFrame)
    {   IEC0bits.U1RXIE=0; //turn off the U1RX interrupts
        readFrame = false;            
        while(indexer < 8 )
        {   
            while(!U1STAbits.URXDA);
                modbusBuffer[indexer]=U1RXREG;
                indexer++;
        }
        if(indexer == 8)
        {
            modbusRecvTask(modbusBuffer);
            indexer=0;
        }                      
        IEC0bits.U1RXIE=1;     //turn on U1RX interrupts       
    }

void _ISR_NAP _U1RXInterrupt()
{    
if(IFS0bits.U1RXIF)
{
    IFS0bits.U1RXIF = 0;    //set the interrupt flag to false
    if(U1STAbits.OERR==1)   //check overload error
    {
        U1STAbits.OERR=0;       //clear error flag
    }
    else
    {
        readFrame = true;        
    }
}    

}

The thing is that it works fine only for the first received frame. After that the program goes into the receiver interrupt again and sets the flag readFrame to true even though no bytes were send and is getting stuck in line:

while(!U1STAbits.URXDA);

I've read some advices to clear the read buffer of the UART in order to prevent the program to go into the ISR again but I couldn't find a way to do it.