"I'm going to tell you" versus "I'm going to be telling you." What's the difference, please ?Please tell me the difference between "I'm going to...

This is a fun question. I'm not sure that I have an answer, but
I have some ideas of about how you might look for answers and what you might
find.

One way that you might explore (in a limited fashion, of
course) the real-world uses of these phrases is to perform an internet search for these exact
phrases. Put quotation marks around the phrase "I'm going to tell you" and search for examples.
Then do the same, again with quotation marks, for the second version of the
phrase.

The results are open to interpretation, of course, but
you'll see that "I'm going to tell you" is used much, much more frequently than "I'm going to be
telling you." You may also note that the second version ("I'm going to be telling you") seems to
be much more closely tied to instruction of one sort or another.

Of
course, you may also find that the differences aren’t all that clearly defined in the end. If you
perform an internet search using both exact phrases (in one search), you will find instances of a
single person using both phrases almost interchangeably and often side-by-side. See Sammiee’s
post (dated August 26, 2010) at http://www.animals-zone.com/cute-baby-animals/, which includes
the statement: “I’m going to be telling all my friends about this website because we all enjoy
looking at pictures of [baby animals]. I’m going to tell my family also! Thanks for this site!!”
Similarly, a blogger (http://candacethetaco.blogspot.com/) uses both phrases nearly side-by-side:
“So I guess by now you're thinking I'm going to tell you what the solution for these problems
are... I know I'm going to pay for this when I have kids one day, and that I'm going to be
telling my kids exactly what my parents are telling me…”

Real-world
examples can provide a useful window (however limited that window might be) into how the two
versions of the phrase might be similar or different.