regular hickup of 1 sec

my vw polo 1.4 gasoline 2002 has regular hickups, of about 1 sec, when all power is gone and the RPM goes down to about 0. Not like when you let go of the gas, then RPM goes down gradually, in this case it's down to 0 at once, for 1 sec, and then up again to normal RMP and speed and normal power. Happens once every +/- 100km. The dealer has read the motermngt system, cannot find the cause. Added stuff to the gasoline to clean the system, did not help. Any suggestions? WA