Looks like a problem that has to do with coverage of the chemistry, possibly with another factor involved as well (e.g. light fogging after incomplete processing).
I'm going to suggest something that may raise some eyebrows, but try developing in trays (instead of drums) at room temperature. Timing is not very critical; take a development time of 2 minutes for starters. Blix time 1-2 minutes; again not critical assuming the blix is fresh or adequately replenished. You'll find temperature is not extremely critical - if it causes visible shifts, they will be the equivalent of around 1cc or 2cc filter pack change (if that, even). All in the dark, of course. Agitate continuously; doesn't have to be very wild, just keep the developer moving across the paper. Use a stop bath of acetic acid (1-2% is fine) between developer and blix.
This would resolve most problems with uneven development. If this works, see if you can get it to work with drums as well.
I get the strong impression that drums tend to create more problems than they solve, at least for some people. But it may just be my preference for the simplicity of trays talking here