I installed a tall, elongated 1.6 GPF toilet 8 years ago. It works
fine....most of the time. About every 100th or so flush, the bowl
fills with water from the tank but for whatever reason it doesn't go
down. I stick a plunger in the bottom, pump it a couple of times and
VOILA! everything goes immediately down. It's then good for the next
100 or so flushes. This has been going on all the time I've had it.

The other toilet (which is nearer the main sewer) has never failed to
flush correctly, and no other drain in the house is backing up. Any
ideas?