I have exactly the same problem as you.
The reason seems to be that I mix up the hiragana. What I would do is get a piece of plain A4, draw squares on it all the same size, photocopy it about 60 times. Then write and write and write and write the symbols up to Ho until you have them so deeply ingrained into your brain, then start to do rows of symbols. Do a row of, say, ra, then a row of ri, then ru etc.
Then do about five sheets of the R +vowels.
It works a treat, and got up to No into my head so much that I cannot forget them. I haven't had much time recently for any Japanese, due to Exams, but it does work.
I got the idea from my local Chinese takeaway. A girl in there was using this method to learn the chinese characters and to perfect them.
It does help to write really good symbols, getting you used to writing symbols almost perfectly.
Hope I have helped.
じゃまた
ジョー