first off, thanks a lot for the input. much appreciated.
should insufficient fixing be the cause, would refixing sort it completely then?
the fixer used was fotospeed fx30 (the one branded as odourless). used it many times with no problems whatsoever. it was fresh (i use it as a one shot) but from a nearly empty bottle i first opened in (i think) march, maybe april. have been wondering how long it could last but served me well until last week, when i developed films in ID11 and Rodinal.
for the agitation routine, i did 7:30 with agitation on the first 10 seconds and then 5 secs every 30 secs, 3 sideways movements left-right within a range of about a foot.
while fixing, i did 5 minutes at 1+9 dilution and agitated the first 10 seconds and after every 90 secs for a further 10 secs (hence agitating 3 times).
also, could it be of any help, the film i developed straight afterwards, reusing half of the dev, came out nicely but pretty stained despite careful routine washing and use of wetting agent.will try to wash again but i fear it could have some unremovable marks due to a little too much handling while loading into the reel (had to do it again 3 times as the reel, which i hadnt used before, seemed faulty. the reel i used instead was still a little damp).
the film did go through x rays twice, but i've done this many times and never had any problems whatsoever. unless oslo-torp airport's x ray machines are different than that of many other places, i can rule the option out, having already travelled with the same film in the hand luggage.
i used 650ml of dev, which is in excess of the 600ml required. i always use a bit more to make sure the whole film is sufficiently dunked. also, the problem is common in both films, which were staked one on top of the other.
i will try refixing a strip of 6 with new fixer either tonight or tomorrow. i loved visiting oslo but am hating having brought back such a bad cold/sore throat, might prefer a champions league game on the sofa, under a blanket than some more fixing...!
once again, thanks a lot for your interest and help!