reci p18 power supply trouble, not enough current

Hi, i have a reci power supply p18, it work but just until 24.1ma and not stable at this current. but at 23ma all work good for lot of hours.
whne i start in input at 0v to 5v the currnet increase from 0 to 3.4v at 3.4v i have 24.1ma then at 3.5v current start to descrease.
i exchange power supply for a spare one i have here (butnot a reci a chiness 150w) and all work good, I tryed until 30ma without trouble them descreaseto 26ma for security.
them i think my tube is good but my reci power supply have trouble. i tried to turn the trim-pot to increase current but i have same result but change in voltage trouble. if i increase trip pot current start to descrease at 3.2v instande 3.4v, ifi descrease trim pot current start to descrease at 3.6v until 3.4v.
any body have idea?
thanks