Turn signals work sometimes

i have a 90 xj and its more or less a beater so i dont really no where to start looking but when i turn on my turn signals either one sometime they work fine and other times they will stay constant and wont blink at all then sometimes it will stay constant for 10 seconds or so then it will start blinking the only time i never have a problem with it is if im towing something any ideas would be greatly appreciated

x2 on the flasher. I also have a 90 and had intermittent problems with my signals, replaced the flasher and haven't had any problems since!