Compiling to ePub

I have recently completed a project in Scrivener and compiled the output to three formats.
1 Kindle
3 ePub.

Kindle and PDF worked like a dream but ePub was very erratic. Line spacings varied widely seemingly randomly, fonts changed unexpectedly and the font size went from small (12pt) to very large (36pt) and back again in a single paragraph.

I work on a Mac with OS 10.6.

Does anyone have any suggestions on how to “control” the ePub output, or tell me what I am doing wrong?