Hi again, rsrasc! I've tested & retested the code and it works properly for me. Some notes: before running the macro, you should select some part of your txt. If you want to process the whole doc, press Ctrl+A. As for attaching a sample file, I don't think it's really needed, just copy-paste the following txt in your doc & run the macro:
Test (test) test (test)
Test (test) test (test)
Test (test) test (test)
Test (test) test (test)
Test (test) test (test)

And the code itself:
Sub Bold_Parentheses_If()
'Bold & color red the strings in parentheses in the selected range.

Dim rng As range

    Application.ScreenUpdating = False
    Set rng = selection.range
    With rng.Find
        .ClearFormatting
        .Replacement.ClearFormatting
        .text = "\(*\)"
        .Forward = True
        .Wrap = wdFindStop
        .MatchWildcards = True
        Do While .Execute And rng.InRange(selection.range)
            rng.Start = rng.Start + 1
            rng.End = rng.End - 1
            rng.Font.ColorIndex = wdRed
            rng.Font.Bold = True
            rng.Collapse wdCollapseEnd
         Loop
    End With
Application.ScreenUpdating = False
Set rng = Nothing
End Sub