ADC12 input channel-to-channel interference?

I'm using ADC12 in single-channel-single-conversion mode on an MSP430F5419 (non-A).  I have two analog inputs:

ch1 has no signal input for 0 counts
ch2 has a small input for 1290 counts

If I continuously read ch1 ch2, all conversions are correct.  If I alternate between channels, ch1 ch2 eventually shows a 0 conversion result (or near 0), then returns to normal.  I'm at a loss.  I've scoped ch1 ch2 adc input looking for low-value fluctuations...none found.  Any help is much appreciated!

Code

//------ Internal ADC Setup --------------------------------------------------------------

ADC12CTL0 = ADC12ON + ADC12REFON;                    // Turn on ADC and reference
ADC12CTL1 = ADC12SSEL_1;                                          // Select ACLK (1 MHz) as ADC clk source
ADC12CTL2 = ADC12PDIV + ADC12RES_2;                  // PreDivide-by-4 ADC clk, 12 bit conversions
ADC12MCTL0 = ADC12SREF_1 + ADC12INCH_0;       // Select ADC reference 1.5V internal, ch0

//------ ADC state machine -------------------------------------------------------------
switch (adc_sm)
{
    //------ Start sample channel -------------------------------------------------------
    case 0:
    ADC12CTL0 |= ADC12SC + ADC12ENC;       // Sample channel
    ticks = SYS_TICKS;                                               // Get system tick count

     adc_sm = 1;                                                          // Select next state
     break;

     //------ Check sampling complete ----------------------------------------------------
     case 1:
     if (CheckTicks(ticks, TPS * 0.1) == true)     // If 100ms sample period expired
     {
          ADC12CTL0 &= ~ADC12SC;                    // Start conversion
          adc_sm = 2;                                                 // Select next state
     }
     break;

      //------ Check conversion complete --------------------------------------------------
      case 2:
       if ((ADC12CTL1 & ADC12BUSY) == false)
       {
            ai_ch[ch].value = ADC12MEM0;               // Save conversion

            ch = ch ^ 1;                                                   // Get next channel

            ADC12CTL0 &= ~ADC12ENC;                // Disable ADC
            ADC12MCTL0 &= 0xF0;                            // Clear ADC channel select
            ADC12MCTL0 |= ch;                                  // Select new ADC channel

            adc_sm = 0;                                                // Select sample channel state
        }
        break;

        default:
        adc_sm = 0;
        break;
}