{enter} not always working

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I'm using Autohotkey V1.1.26.00, Unicode 64bit, on two different Windows 7 systems, to send keystrokes to Autocad.
My script has been functioning flawlessly for years on one system, but not on the other.
The script is quite lengthy, but for the purposes of this discussion I have isolated the problem down to {enter} not always working.

This is what I'm using to send "ENDPcrlf": f6::send ENDP{enter}
I have sent the output to notepad++ so I could check for special characters.

On one system I get what I expect: ENDP followed by crlf for every press of the F6 key.

On the other system I get this:
ENDP
ENDPENDP
ENDPENDPENDP
ENDPENDPENDPENDP
ENDPENDPENDPENDPENDP
ENDPENDPENDPENDPENDPENDP
ENDPENDPENDPENDPENDPENDPENDP
ENDPENDPENDPENDPENDPENDPENDPENDP
ENDPENDPENDPENDPENDPENDPENDPENDPENDP
ENDPENDPENDPENDPENDPENDPENDPENDPENDPENDP
ENDPENDPENDPENDPENDPENDPENDPENDPENDPENDPENDP
ENDPENDPENDPENDPENDPENDPENDPENDPENDPENDPENDPENDP
ad infinitum

where each ENDP is a press of the F6 key.

The only thing I can figure out is that it's not random.
Anyone have any suggestions where to look?

thanks