#VALUE! Error - Unknown cause!

Further to my post here, I've managed to sort the circular reference error, but now I have a new issue I can't seem to resolve.

I have a custom formula, and I originally set variables with VarType's I thougt appropriate. Since I kept getting #VALUE! I started debug.print'ing each variable with Vartype(variable). I've since changed each forumla variable to this type reported.

It seems to re-calculate OK when you are that page, but when you go to another page, it returns back to #VALUE!.

If I do "Evaluate Formula", it calculates the number but then returns #VALUE!. The Evaluate Formula last step is: IF(FALSE,#N/A,5947.7245) then goes to #VALUE!

Using "Error Checking" it says "A value used in the formula is the wrong data type". I've check each value and it's all correct. The only one it could be is the range stfPayNum. I can't figure out though, what is should be if not a range?

I've attached a sample of which shows you. To replicate the problem, go to One sheet (Teaching or Support), hit "Calc Now" then go to the other sheet. You'll see they are all #VALUE!'s again.

I've tried with, without application.Calculate/CalculateFull/CalculateFullRebuild but all the same result.

Any help would be much appreciated.