Hello Dag,
That, TURNING IT SELF OFF, happened to my Explorer as well. You may want to try this.
Firstly try another battery , like maybe the alkaline battery pack.
If that is NOT the solution then disconnect the coil and wait and see, at the point that this problem is most frequent, if it still turns itself off.
If it dose NOT turn itself off then, you may wish to try another coil. If after you have tried another coil and the problem is still there then,I can tell you that at this point, from my little expereience, it is most likely that the internal power supply circuit is faulty.
It can be repaired and cost me about US$70.00 dollars to have Minleab do it prompltly. I had to pay as I voided warranty before it ended.
I am sure it would have been covered under warranty if it still existed.
Have a good one.
HARDNOSE