M.,
You might want to get the 'shiver' on video and get to a pediatric neurologist. Is it recurring in clusters?, you mentioned it's becoming more frequent.
Of course, babies make all kinds of funny movements for various reasons and it's likely just that, but I have to share something with you ...
My 4 yr old daughter has a seizure disorder and at first I was the only one who could see it. The pediatrician - and even the ER doc wouldn't believe me because her 'episodes' are so very quick and she has no disorientation before or after, and.. at the time if you were not looking right at her for at least 15 minutes you would miss it (impossible during a typical Dr's appointment). She would have a little shiver and her head would drop for a fraction of a second and then she'd be fine again.
I Don't mean to frighten you, and it's probably not the case in your little one, but seizure disorders can be extremely varied - differ from person to person. We finally took a video of our daughter and after five visits to Doctors and ER, made our way to a pediatric neurologist who immediately acknowledged that these were, in fact, seizures after viewing the video. Her EEGs came back normal because it is occurring very deep in the brain. My little girli is seizure-free with just a small amount of medication and she's just like every other normal 4 yr old.
Don't give up - if the doctor brushes it off...you know when something isn't right and if it might possibly be seizures, there is potential for more serious seizures and this can cause brain damage if left unchecked.
I hope I didn't cause you more worry - trust your instincts!
W. :-)
P.S. There are only a couple of pediatric neurologists around here - we see Dr. Priti Singh she is based up in the burbs but has an office in Crest Hill where she sees patients twice / week (she is awesome) ###-###-####.