SES light is continuously on except for shortly after a disconnect of battery, but then resumes being on within a start or two. No code is being issued, had it checked multiple times at parts store. Issuing a reset from the reader has no effect.

Any time my Blazer's been sitting for a day or more, when I leave town and steadily accelerate towards 55/65, around 45-50mph the RPM will fluctuate rapidly between 2500 and 3500 until I've been steady at 55/65 for at least 5 minutes. Particularly notorious in winter, often occurs every start when it's below 35 out, regardless of whether or not it's been sitting.

 

Have you checked for +12V power at pin 16 of the DLC (data link connector)? Have you tried getting it scanned with a full feature scan tool? The SES light being on means that the PCM has stored a code.

As far as the symptoms go, it sounds like your transmission is having difficulty deciding which gear to be in, 2nd or 3rd. A problem with the TPS sensor and/or circuit can cause odd shift behavior as can a dirty MAF sensor, but both should exhibit other abnormalities as well.