1. The generator deliver a constant Amplitude (Voltage) but not a constant Current. This current depends on the load. The load is yourself and the electric properties if your body is not a constant therefore the current will change over time. (The contact between the pads and your body is also a variable).
So this is normal. You can overcome this by using an external constant current source or by using a Resistor in series to your Body that will reduce the max. current that can flow.
2. Another possibility is that your tens cable is damaged.
To test the cable I do the following:
1) Load Killing - JW Preset.
2) Load Abdominal Cramps Program - it starts with 72hz which is a very low frequency you can feel.
3) Go to Settings and enter a Ramp Up of 20 seconds (Make sure Reduce Amplitude is not ticket)
4) Start the generator and get ready to hit hold!!!
5) Before the voltage gets too high & uncomfortable hit the hold button.
6) While you can feel the frequency quickly check your cable for loose connections, and move it around to check for any breaks.
For more details, please check the link: