Logically, it looks liek your code checks out so the only things that come to mind are:

1) I am assuming (and hoping) that you have your ncurpos defined as a module level variable. If it doesn't then it won't retain your value as you call the function repeatedly.
2) Your code would break down if your cP string only contained one Chr$(13). For example if your cP was equal to:

abcdeXYfghijk

where X=chr$(13) and Y=chr$(10), then the first time it would read strTemp as abce but the second time it would return nothing since it can't find another instance of Chr$(13). Try this if you want to make sure of your input file.

Instead of reading from the file and writing it to sData, instead write it to an output file. Then you can use a 32-bit text editor to check the file to make sure that your chr$913)s and chr$(10)s are actually there and in the right place.