28335 SPI Transmit Problems

Hi, I'm new here so I hope I'm going about this correctly.  I have searched to see if anyone else has had a similar problem to mine and I can't seem to find it, so any help you all could offer would be greatly appreciated!

I am using the 28335 and running a timer interrupt every 20 us (I am trying to generate some sine waves to output to an audio amp).  I am using the SPI module to communicate with a DAC, but I can't seem to get it to run in real time.  Currently if I send out a single value to the DAC, then set up a breakpoint immediately following the transmit function, the program will run to the breakpoint, send that value and the DAC will properly output it as expected.  (I have set the SPIPRI bits to run the spi freely on halt).  The problem is, when I run the program at full speed the values never seem to get sent.  If I halt the prgram at any point the most recent SEND value is transmitted, but ONLY after a halt or breakpoint.  During the free running of the program the DAC stays frozen at whatever previous value was given to it at the previous program halt.

I know the DAC is working, or it wouldn't update at all.  I've tried sending it a simple square wave, but still no luck there.  The amp is working fine, because there is a very audible 'pop' whenever the program is halted and the output jumps.  I have checked, double checked and triple checked that the spi baud rate is slow enough for the DAC (it's an LTC1661 if that helps), and it is.

I'm really not sure what I'm doing wrong.  Any suggestions?  Here is the relevant code:

(main program timer interrupt loop)

{

<stuff here>

send = (send | 0x9000);

    SpiaRegs.SPIFFTX.bit.TXFFINTCLR =1; //do I need this?  It doesn't seem to affect anything since I don't actually need to read in any data
     // Transmit data
   spi_xmit(send);

}

void spi_init()
{   
    EALLOW; //not sure if I actually need this, but just in case
    SpiaRegs.SPICCR.all =0x000F;                 // Reset on, rising edge, 16-bit char bits 
    SpiaRegs.SPICTL.all =0x000E;                 // Enable master mode, delay phase of clk
                                                 // enable talk, and SPI int disabled.
    SpiaRegs.SPIBRR =0x004F;                    //originally at 0x007F   ,slowed for DAC            
    SpiaRegs.SPICCR.all =0x009F;                 // Relinquish SPI from Reset  
    SpiaRegs.SPIPRI.bit.FREE = 1;                // Set so breakpoints don't disturb xmission
//        SpiaRegs.SPIPRI.bit.SOFT = 1;
        EDIS;
}

void spi_xmit(Uint16 a)
{
    SpiaRegs.SPITXBUF=a;  //changed from SPITXBUF
}   

void spi_fifo_init()                                       
{
// Initialize SPI FIFO registers
EALLOW;
    SpiaRegs.SPIFFTX.all=0xE040; 
    SpiaRegs.SPIFFRX.all=0x204f;
    SpiaRegs.SPIFFCT.all=0x0;
EDIS;
}

Thanks in advance everyone!

Jon