Excel 2007 : copy formula and cell reference does not change

I am currently on Excel 2007.
I have one spreadsheet that I've used a very long time for my personal budget that has been around since at least when I was on Excel 97. It has occassionally had some issues and I have copied the data out and saved as a new spread sheet. Since getting to Excel 2007, I've noticed that it doesn't always change the cell references correctly when copy or cut then pasting. i.e.
Cell P1153 contains =IF(Q1153="Y",O1153-C1153,O1153)
If I copy this cell to P1154, often but not always, it copies as:
=IF(Q1154="Y",O1154-C1153,O1154)
keeping the original reference on the 3rd variable as if I had an absolute reference????
If I pasted down to P1160, it would still be C1153 in that position.

This is the only spreadsheet that I've noticed this occurring.
I manually fix the formula and it calculates fine...?

I asked our help desk here after I managed to prove to them that it was not my imagination, but they didn't have any ideas. Anyone have any suggestions of what could be causing this and how I can correct it?

I noticed this same problem in an Excel spreadsheet my manager sent to me where if I copy a cell containing a formula that does not have any "$" dollar signs, the formula does not update the relative cell references. There must be a way to change the default cell references.