Goal Seek Issue

[frrbm4]

I am having an issue when using Goal Seek. It searches and eventually ends on a random number that is incorrect and a constant rather than keeping the original formula in the cell. Additionally, once this happens I cannot Undo and I lose the original formula.

Has anyone else come across this before? I have sent the doc to a colleague in my office and he has no problem using goal seek in the exact same spot.

Thank you in advance for any help you can offer.