Try an ovulation predictor
kit. Because your cycles are irregular, start the strips around
cycle day 7 (with day 1 being the first day of your menstrual period). Test around the same time every day, preferably late afternoon/early evening. This can help you determine the best time to have intercourse.
Also, if you haven't done so already, you might want to seek the assistance of a fertility specialist. Sometimes there are things they can do to regulate your cycle and help you out.