[Premium-Rx] WJ8888 - Strange fault

[RichardReich at aol.com]

Hello all.
 
I am in the process of restoring my WJ8888 to working function, and I have  
the comms datastream up and running again.
 
However, there is a strange fault  - which has taken some finding  (weeks).
 
There is one particular CMOS logic device (2-input NOR gate) that will  
only work with the scope probe attached to its output pin. If the probe is  
removed, the device slowly stops working after about 30 seconds.
The data becomes spikey, and then just fades away - as if the chip is  
slowly running out of gain.
 
Once the probe is replaced, the device works perfectly once  more - 
instantly. The scope does not have to be attached to the probe  to have this effect.
 
I have changed the chip (twice), and the one after it (twice) - it  makes 
no difference - the problem persists. Its almost as if the chip now  requires 
some capacitive loading to work at all. I am reluctant to add a  cap which 
wasn't in the original design to fix this.
 
The circuit is not fast (a few kHz).
 
I wonder if anyone else has seen anything like this - and has an  
explanation or solution?
 
Never seen anything like this behaviour before with logic devices. 
 
 
Thank you....   Richard