i dont know if its a relay issue, but i just had my 2000 RX in yesterday to fix this same issue, mine would work on and off, just as you described, untill last week when it just got stuck down, i drove around for 5 days with the window down!!! they had to replace the motor.
ps- my issue was when it was down, and i tried to put it up, it would go down, then up, then down, and finally up again. just wondering if yours is similiar..