You may right about the stray or false triggering as mentioned in the data sheet.
Now try putting a little delay after the PULSOUT to give the device time to complete the transmit function before listening for an echo.

Code:
distance:
    PULSOUT trig,1      ' each count = 10us
    PAUSEUS 120      ' Wait for transmit pulse to complete
    PULSIN sens,1,dist  ' will time out in 655ms if nothing received
    dist= dist/15 'convert to inches
    PAUSE 100
RETURN

END