MSP430FR2433: EUSCI_B_I2C_masterSendMultiByteStart(I2C_0_BASE, cmd); library call sometimes hangs.

Part Number:MSP430FR2433

I am using a MSP-EXP430FR2433 as a USB serial to I2C Master device. 

Sometimes when I start, it hangs in the EUSCI_B_I2C_masterSendMultiByteStart() library function. 

void EUSCI_B_I2C_masterSendMultiByteStart (uint16_t baseAddress,
    uint8_t txData
    )
{
    //Store current transmit interrupt enable
    uint16_t txieStatus = HWREG16(baseAddress + OFS_UCBxIE) & UCTXIE;

    //Disable transmit interrupt enable
    HWREG16(baseAddress + OFS_UCBxIE) &= ~(UCTXIE);

    //Send start condition.
    HWREG16(baseAddress + OFS_UCBxCTLW0) |= UCTR +  UCTXSTT;

    //Poll for transmit interrupt flag.
    poll_loop = 0;
    masterSendMultiByteStart_fail = false;

    while (!(HWREG16(baseAddress + OFS_UCBxIFG) & UCTXIFG)) 
    {
      poll_loop++;
      if (poll_loop > poll_loop_longest) poll_loop_longest = poll_loop;
      if (poll_loop > POLL_LOOP_ABORT) 
      {
        masterSendMultiByteStart_fail = true;
        break;
      }
    }

    //Send single byte data.
    HWREG16(baseAddress + OFS_UCBxTXBUF) = txData;

    //Reinstate transmit interrupt enable
    HWREG16(baseAddress + OFS_UCBxIE) |= txieStatus;
}

the original while was 

while (!(HWREG16(baseAddress + OFS_UCBxIFG) & UCTXIFG));

I added the counts to give me an idea how long it will hang. I gave up when the pool_loop number hit 4,000,000. 

Sometimes this will run for hours without a problem.

However I took it apart put it back together and it hangs on that while. I have not completely figured out the pattern to get it working again but after fiddling with the reboot enough times on the slave, the master, and the serial program on a PC, it will start working.

I am beginning to think it is the slave not responding to the start and address with a ack. 

How do I debug this? 

Is there some sequence to put the I2C lines in a known state like if it was reset? 

Unfortunately it was working at the office, when I went to the customer's 200 miles away, and tried to start it up it would not start. And I do not have all my tools here that I have at my office. 

Any suggestions?

Kip