Never trust a PIC at boot, it's output level is hard to predict. you should set it yourself at the top of your code and add a little delay
Code:
    PORTA = 0
    PORTB = 0    
    trisa = %00010000
    trisb = %00000000
    PAUSE 50 ' settle time 

loop:
It's not a bad idea to enable the Power Up timer in your Configuration fuses as well.
Code:
@       __CONFIG _XT_OSC & _PWRTE_ON & _WDT_OFF & _CP_OFF
http://www.picbasic.co.uk/forum/showthread.php?t=543