I may be missing something here. You've posted:
"Right now I can sort by Month name and get running totals for that month"
If they are running totals, then the balance from the previous month is automatically the starting balance for the current month unless you've selected the "Restart Summary for each sorted group option." If you have, you can either clear this option, or ( if you need the restarted running total in other areas) define a second summary field with the running total option selected, but don't select the restart option for it.
You are indeed correct.
The real problem it turns out is that I had the wrong field in the sub summary part. My God the years are not being good to me.
Thanks for your time. Sorry for the trouble.