Is the 7474 a
'dog'?  I've know a few TTL parts that were pretty bad,
 mainly counters though.  But a latch? 
 This reminds me of a similar problem in the JC80 systems, where 74170
 chips would fail more often than anything else. I'd sure like to know what
 the cause of that was... 
 
I can remember replacing many 7474s in the RK11-C when I was getting that
going. Oh, and a 7482 adder, which was impossible to find [1]...
More recently, I've had a lot of 74Hxxx parts (and yet more 7474s) fail in
HP98x0 calculators. I guess after 30 years I can't really moan...
[1] I do mean _iompossible_. I ended up using part of a 7483 that I stuck
to the flip-chip PCB dead-bug style and wired to the holes where the 7482
should have been. Runs fine...
-tony