One more "interesting" observation.
Sometimes when flushing, the solids will first appear to go down the
trap, then "regurgitate" up into the bowl and then drain back down
again. This does not always happen.

Does this indicate anything?


YES! It indicates the amount f water entering the bowl is marginal. So
your not getting enough OMPH to get past the trap inside the toilet and
start the suction of the flush. How toilets actually flush is pushing
enough water by rim holes of bowl ad the jet in the bottom of the bowl
to start the flush, vacuum finishes the job.

I am sure acid would fix it!

However since you like I was are skeptical.....

For the next week NEVER flush by pushing the lever!! NOT ONCE!

!!!!!ONLY FLUSH BY DUMPING A FULL BUCKET OF WATER INTO THE BOWL!!!

IF you get no clogs this PROVES its needs acid. and honestly its not as
dangerous as it sounds.

Again If the toilet flushes good using a bucket of water then the
plumbing downstream is fine.