unpredictable barking at people

Question
I have a 2 1/2 year old, neutered terrier mix (sire was a soft-coated wheaten terrier, dam was an unknown non-terrier mix).  He is more playful than affectionate and does not care much about praise.  Food treats seem to be his only motivator as far as training; "good boy" means nothing to him.

The problem:

Some people who greet him get barked at.  This appears (to me) to be playful barking, but seems threatening to folks who don't know dog body language and is annoying in any case.  It continues until I call him away or remove him from the person's vicinity.  Some who greet him do not get barked at.  People who ignore him almost never get barked at; although if the person has a hat, backpack, bicycle or unfamiliar object, that may trigger the barking response.

He has always displayed a certain level of fear/nervousness around new objects.  Umbrellas, bikes and skateboards, and hotel maid-carts are known culprits.  The response to new objects (without people involved) is usually a nervous-submissive pose (no barking or whining) which I can remedy by moving away from the object and then rewarding the new, happier body language that results.

However, the barking at (some) people is both less predictable and harder to stop once it begins.  It seems that tall people are more likely to get the barking response, and those who actively greet him (with eye contact or by reaching the hand toward his nose) are also more likely, though not certain, to trigger it.

Rarely, as in the case of a stranger walking a bicycle through the dog park, I can predict this barking and stop it before it begins by calling him to me or otherwise distracting him.  If I could at least predict this response, I might be able to alleviate it.  What stymies me is that some people are greeted normally with no barking or submissive/nervous response at all, while others get the full-lung nonstop barking.

Answer
He may have been less than adequately socialized to people when he was between age 8-16 weeks (pups need hundreds of contacts, and many don't get that).  So, he's likely more comfy with types he's familiar with, OR he's more fearful if someone is big, loud, stares at him, approaches frontally, or reaches for him.  I would suggest you have a look at this site:http://fearfuldogs.com/ because I think that this is a fear reaction and you need resources to help deal with that.  Punishment is a big no-no, and classical conditioning is a big yes-yes:-)