Gil,
Hmmm...this is a good one. First of all, you may want to try using STRTOFILE() to do your debugging in this situation...I find it the most reliable in odd situations where running it normally has one behavior and stepping through the code has another.
I would definitely place a STRTOFILE in your SumTotal() method to see what's going on. Also, is it possible you're running into LockScreen issues?
Regards,
Kevin McNeish
Eight-Time .NET MVP
VFP and iOS Author, Speaker & Trainer
Oak Leaf Enterprises, Inc.
Chief Architect, MM Framework
http://www.oakleafsd.com