Finally got to check the cruise control cable this afternoon and found that when the throttle is open the cable becomes loose and may slip towards car back. If the throttle is open wide enough the cable can move excessively and becomes stuck to the bracket on its way back when the throttle is closing, hence causing the throttle to remain open, as shown exaggeratedly in the first picture.
But this would not happen if the ridge (red arrow) faces right, as shown in the 2nd picture, when the cable is in its original position, as shown in the 3rd picture, because the ridge will be forced by the spring tension of the cable to the right side, stops at the bracket from the other side and prevents the cable from moving out of the bracket.
So I think my wrench installed the cruise control cable with the ridge facing the wrong side, causing the cable to slip out occasionally at high RPM when the throttle is open wide. This also explains why I had the problem more frequently when driving uphill as the cable slipped backwards more easily thanks to gravity.
Now I think the case is closed unless the problem shows up again.
