Overdue bearing maintenance is not going to produce a clunk at a fixed point every time, but there is a likely candidate. Fixed cups sometimes loosen enough to shift under pressure, but not enough to detect when doing the typical "is the bb loose" check. It's common to feel it mainly on the right, as there's more direct pressure on the fixed cup from that side.
Remove the right crank arm and put a wrench on the fixed cup (tighten counterclockwise). If it does move that's the problem. Before cinching it down loosen the lockring and back off the adjustable cup. Now tighten the fixed cup solidly and finally adjust the bottom bracket. Of course it can't hurt to overhaul the BB.