With masculine nominatives that end with 'a' (ASRONAUTA, DENTYSTA) what are the instrumental versions?
astronauta - astronautą
dentysta - dentystą
(this rule appeals to most nominatives ending with -a)
I don't know how are the rules, but z_darius's rule with -IEM/-EM doesn't work with all jobs. Unfortunately I can't guess what the real rule is.
fizyk - fizykiem
strażak - strażakiem
I suspect that the consonant that closes the word is important here.