Nissan Repair: Nissan 300ZX ignition circuit, nissan service manual, mass air flow sensor

Question
Hi Von:
My 1986 Nissan 300ZX (172k miles) started bucking same as it did when my mass air flow sensor failed.  But when I checked the error codes, it shows #21 for "ignition signal in primary circuit not entered 10 times".
(Also shows #13 for cylinder head temp, but all components check out and after clearing the codes that has not re-occured).
I followed the procedure in the Nissan service manual that I have, and the only item I suspect is the "power transistor".  But before I shell out $150 can you verify how to test it?  My manual says to test continuity from 1 to 3, 2-3, 2-1 so I'm putting my red lead on 1, black lead on 3 (in the first scenario)(then red on 2 black on 3, red on 2 black on 1).  In this test, I do not show continuity from 2-1.  Yet the car is currently running fine (though I'm afraid to drive it).
This service manual lead me astray once, so I don't have 100% confidence in it.
The distributor cap and rotor look worn, but they are only 6000 miles old.  I suspect weak spark causing extra wear?
Any suggestions?  My thought is to replace both the power transistor and the coil (for good luck).
Does that sound right?
Thanks!!!
-Mark

Answer
It is the power transistor if you read what you wrote me.  It is not necessary to change the coil.  But, with the years on the car and the miles it would not hurt.  Basically, the power transistor is going bad and it does not all fail at once and that is why you are getting partial continuity.