[time-nuts] May I ask for your advice on a faulty 5370B?

[S McNamara]

G'Day All,
I just purchased a 2nd hand HP5370B counter.
It was last calibrated in 2002 but fails the operators checks.
I have run tests 1-21 and only a few fail.
In step 13 the display indicates 90.9*ns
In step 15 the std dev is 20.* to 16.*ns with a min of 90.9*ns and a max 
of 91.*ns
In step 17 the stop period is 91.*ns and the start period is 8.91ns
I have measured its ref output with another counter and it seems stable 
and 10MHz.
So I am seeking ideas on where to start to fault find, any guidance 
would be much appreciated.
Regards Scott