A little more haplology.

[R. Rankin]

>According to my dictionary, to verb is "lighten" which I guess means the
correct verb form is "It is lightening"--referring perhaps to the sky
being lit up by a flash.

That's the form I get from Okies, but I never met anyone else who said it.  Dictionaries are made up by people like us, so I suspect this is just some English teacher's wet dream.  :-)

If one needs to be forced to utter it, try it in the combo "It's sure thundering and L___ a lot!"

Linda's example with Dakotan -$iN-$in is certainly interesting.  I take it that other stative verbs DO accept that suffix.

Bob