Just a couple of ideas.

1. You have no pause in your main loop so you are polling the DS18B20 extremely fast. You may want to try a Pause 1000 (1second) in between you gosub start_Convert and your goto mainloop.

2. Your If/Then LED logic in start_Convert will only light the LED when the temperature is below 70 degF.
Code:
if F_temp =>70 then
low LED
else
high LED
endif