It sounds like some of your cells are formatted as text--or the formats are
being changed to text.

For the summing problem, you could try this:
select an empty cell
edit|copy
select the range to sum
edit|paste special|check Addition

As for the formulas changing to a Text format...

Excel likes to help.

Try this on a test worksheet.
Select A1 and hit ctrl-; (to put the date in the cell)
now select B1 and type:  =a1

Notice that excel changed the format of B1 to match the format in A1.

Now format D1 as Text.
put ASDF in D1
put =D1 in E1
You see ASDF.

With E1 selected, hit the F2 key and then enter (to pretend that you're changing
the formula).

Excel has "helped" you by changing that cell's format to text.

I don't know of any way of changing this behavior.

I just select the cell, and reformat it to General (or whatever I wanted).  I
hit F2 and then enter (to reenter that formula).

Sometimes this feature is nice, sometimes it ain't.