Code quit working - will not format key text

Many thanks to those who manage this forum - you provide a great service. Here are the details of my issue:
System: Mac OS X, Macbook pro, Office 2011
Spreadsheet: 5 columns, variable rows but always a header row. Column C holds key words. Column E holds concatenated text that includes at least one instance of the key word in Column C.
Objective: Search in Column E for the key word that is in Column C and format all instances of the key word only in Column E with bold, italic and underline.
The sub below ran OK several times but quit working now and will not format the words.
Any and all help would be greatly appreciated. Thank you.

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deleted 1/20: code tags applied by mod

Please take a few minutes to read through our rules, specifically the 1 regarding code tags. I have added them for you - this time

The only problem I found is:
PosF = InStr(PosT, Cells(i, 5), aWord, vnTextcompare)

should be:
vbTextCompare

Otherwise the code ran without error (Windows machine).

No solution yet but i extracted the following code to act on a single selected cell. The code below successfully formats the key word (aWord) in the selected cell. BUT, this will not work in a loop like the code above. The code runs without error - also a "step through" shows no error. However, the "Characters" statement does not change the formatting. Also, I ran a different code that has a loop and does cleanup as a test and after running the cleanup the single cell code below would not work either. Any ideas about what might be happening in a loop that would do this? I read that the Characters method only works if the object supports rich text. Could the loop effect this? Thank you.

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