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It looks if the Gun Current can run at (two) different levels. Dose rate can differ more than 50 mu (photons).
Gun current can change with gantry angle.
Enter a high current in Gun stby item 580 (7 or 8 Amp).
Measure with an AVO the voltage over the Gun cable.
Over the outer screen 140 mV, inner lead 70 mV.
Sometimes you can feel warm spots on the cable.
Voltage drop over the cable can vary if cable is moved.
If the Voltage of one or both 'wires' is higher than 200-300 mV then the cable forms a temperature and movement dependent resistor and any adjustment is a hopeless and very frustrating job! Replace ASAP! (We had this phenomenon several times.)
After replacing of the gun cable, check X Gun servo and PDD.
Check for poor Gun servo operation. (TSD)