My guess is that the turn signal is shorting somehow accross to the brake light, and sending power back up the brake wire....which "tells" the Cruise controll that you've just hit the brakes...Of course, this signal causes the cruise controll to "cancel"
Also, since it is doing this with the cruise controll off...I am VERY sure that it is indeed sending the + signal back up the brake wire....As this is causing your torque converter to un-lock at highway speeds. (The RPM flux you are seeing)
Do you have aftermarket signals or LED conversions on any bulbs? Also could be trailer wiring as mentioned....
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