Considering a longer stem for my flat bar hybrid with rear hub drive assist. It feels a little jittery when I take one hand off. Specs say it is:

Stem ( C:31.8mm ) ALLOY 6061 A-HEAD STEM.EXT10°), 90mm

Do I need a replacement that is "A-head?" I don't know what that is.

Also, are adjustable angle stems reliable? Seems like a good thing if they hold up well.

Thanks for any advice.

A Head is just threadless. It's Dia Compe's brand name. Changing the the stem length probably won't solve your 'jittery' feeling.

Also, adjustable stems may be reliable in the sense of very few failures, but almost all of them make some kind of noise and become annoying.

I have adjustable threadless (general term for Ahead-style) on a couple of bikes and have never had problems with noise. Once your experimenting is done and the riding position is "set" you can find a fixed stem with those specs and dispense with the adjustable or just keep on with it. Removable face plates make the stem swap quick and easy. Good luck!

When Dia-Compe came out with the first 1-1/8" threadless headset, they called it the "Aheadset" (tm). As it took over the industry, mostly for ease-of-manufacture reasons, (yes, I'm aware that there are "lighter, stiffer, better" reasons to prefer it, but the "only make (or buy) one fork and have the shops cut them to length" argument was the standard-changer,) there was a period where bike manufacturers called their 1-1/8" threadless fork / headsets "ahead-style", so that they could have the new hotness without having to buy a Dia-Compe headset.

TL;DR: It's 1990s marketing-speak for "1-1/8" threadless headset."

--Shannon