Question
St Augustine lawn in sun & shade areas very spongy  and
bumpy when walked on.  no apparent mole tracks. Appears to
be a seasonal problem in winter months. No recent rain or
over watering.  Curious if we have an insect problem.

Answer
This does not seem to be an insect or fungus problem because you do not describe any adverse effect on the appearance of grass itself. I believe that you operate a watering system and maybe there is an underground leak somewhere. This would explain the sponginess. Moreover, I am guessing that in the winter months you operate the system more frequently and the problem is more noticeable during this season.