Missed periods for more than 2 months...But not pregnant.?

I have irrregular periods from the beginning...All said that it was ok and it will be normal after some time...now i am married for 10 yrs..have a child, but still have irregular periods...last i saw my periods on Dec, 25 and then i got it on March 12.. Whats thiss? when will it become normal..Now i find it very difficult since i cant calculate my peak days and also keep hoping that i am pregnant and thn realise that i am not..whats the remedy to this ? or is there any solution to find my ovulation day? One of my friend said that it can be identified by checking the body temp. daily and on the ovulation day it will be higher than normal ...is it true.. any expernces out there..Pls. Help..