Nissan Patrol Push rods coming out of position

Hi my name is Iorkamave, and I am new to this forum. I have a 1997 Nisan Patrol Y60 model, in-line 6 cyl TB42S engine, only 78000km on the clock. In the past two weeks, I have the push rods for cyl 5 & 6 valve tappets have come off position leading to rough running on only 4 cylinders. I have had the tappet clearances checked, and the rods do not look bent. This problem has happened 3 times in two weeks

Has anybody out there experienced this type of problem, and what is the solution?