Engine "surging" at 2500rpm?
 

I have a 2000 S10 4wd (4dr) Blazer, that slowly over the past year or so has been exhibiting a strange issue.

When I accelerate (usually at the point of 30mph-55mph) the rpms seem to fluctuate by about 400rpm, essentially surging constantly as the speed climbs. Eventually when I hit a highway speed (probably in the last gear), the issue is gone and all seems normal.

More recently it started doing this while stopped at traffic lights. Every 8-10 seconds while sitting, the rpms will surge up by about 300rpm then go back to the normal idle.

Any ideas? Thanks in advance.