I'm sorry, I take it back.VeluCherry said:I...guess that makes sense - you learn something new everyday.^^°NezumiiroKitsune said:Throbbing is the present participle of throb, and still a verb. It doesn't qualify the noun, so it isn't an adjective. Nor can you have an adjective with the suffix -ing.
I guess since English isn't my first language I shouldn't rely on online dictionaries so much... (http://www.audioenglish.net/dictionary/throbbing.htm)
In the case of "A throbbing headache", it changes the quality of the headache (I think!), and so is a non-comparable adjective. Like your site said, it's very rare.
Also sorry for being bothersome about it, I just really like language.
I learned something