First, to make the total hours worked field work, change it from this:
=Sum([txtHoursWorked])
To this:
=Sum([HoursWorked])
You want to sum the field data, not the name of the field. Give me a few minutes to figure out the flickering. Something is in an infinite loop. I'll post back if someone doesn't beat me to it first.
EDIT/UPDATE: The problem is in how you're linking your subform to your main form with the Master and Child Fields. I don't want to just spill the beans on what you've done here, but investigate that on your own and you should figure it out.
Hint: Note that if you remove either Child or Master reference from the subform, the flickering stops, but the link also seems to break. How would you fix that? How do you properly link the two using Master/Child links?
There are an awful lot of posts on this forum referring to how to do that, and in the long run, leading you in the right direction is better than just explicitly saying "Do A, then B, then C".
EDIT/UPDATE 2: To avoid the wild goose chase, here are some more direct hints:
1) You shouldn't link child and master fields on calculated fields. Things might blink unexpectedly, as if they're in an infinite loop (but are instead constantly 'calculating').
2) While the references to columns in a combobox are zero-based (column(0) = the first column, column(1) = the second column, etc.), the bound column property is one-based (bound column 1 = the first column, etc.). Confusing, yes, but good to know.
3) A combobox isn't a calculated field, unlike a certain parent/child combo you currently have.
Hopefully, that should do it.
EDIT/UPDATE 3: The attachment has it corrected, but there might be a password involved. Please try to get it on your own first. If you're really stuck, I'll repost the attachment sans the password. (Feeling a mixture of evil and compassion, sort of like my GF...

).