Excel 2010 copy paste malfunction

In Excel, imagine the following numbers in these cells:

A1: 5
B1: 6
C1: =A1+B1 [Displays 11]

A5: 7
B5: 8
C5:

If I copy cell C1 and paste into C5, rather than copy the formula, which is what I want, it only copies the value of 11. I am specifically not doing a paste-special as values. One strange thing that I have noticed is that sometimes copy-paste works fine but it seems like after a few minutes it always breaks as I have just mentioned. I know it is broken because when it works correctly, the copied cell (C1 in this example) remains highlighted and the border of the cell has the "walking ants" around its border. When Excel is not broken, the walking ants cease.

I have tried to uninstall and reinstall excel but the issue came back rather quickly.

Does anyone know what might be going on? I use Excel 2010.

This appears to have solved the problem. Thank you very much.