And further to my earlier suggestion to check the value of the 18 Ohm
resistor in series with the variable pot (these are small carbon
resistors which can drift high with age) I see that in mine at least
there /is/ an adjustment for the <4nS range which is /not/ mentioned in
my manual. It's a small ceramic trimmer (10 pF?) just to the rear of the
variable pot and presumably replaces the fixed cap in my manual. My
serial is B033902.
Mine goes up to 270 MHz so I'm not about to play with this but if you
find this trimmer in yours it should get you up to spec.
But with so many suggestions that have been offered that's it from me :^)