Are you temping orally or via your girly bits? If orally and you sleep with your mouth open this can have an affect on your temps. I found this out and now temp downstairs

since then I find that my temps are fairly stable but still vary mid to high 36oC
Are you using Fertility Friend? If so, they will only confirm ovulation after 3 days of higher temps (following a dip). I'm not 100% sure but I think you ovulate on the day of the dip (that's what my charts do anyways and I also use an OPK test).
Everything I have read says that you "should" ovulate around the same time all the time - But I'm all over the place from CD14 to CD19
As for tell tale signs for ovulation I go by my temps and CM and also get particularly interested in BDing