Engine loss of power?

As it turned out, the O2 sensor on the top of the catalytic converter was loose and was showing signs of wear and damage. This condition led to it sensing additional oxygen in the exhaust which it compensated for by increasing the mixture richness. Thus, my engine was being flooded by too much fuel and "surging" as a result. This also explains the decrease in fuel efficiency.

The above insight cost a pound of flesh, however, but I appreciate Bruce's advice and the help of the forum.