HMMMMMMMM.............
Nikasil issue was a problem, but most engines suffering from it will well be showing signs of it by now, it doesnt tend to show up suddenly on engines the age most are now...........
3 litre V8 can be a problem, p/s problem would usualy be down to pipes from unit, not unit it self unless seals are pukkaroo'd.
The 3 litre tends to have more issues with rocker & head gaskets than 4 litre, rough idle can be due to oil weap from rocker covers getting into sparkplug & spark unit.
All V8's can suffer from pcv ( ? ) valve degridation, leads to rough running & poor idle, also crank sensor's tend to be a common problem.
My last 3 litre pack a sad due to afm giving up the ghost, easily sorted.
Sticky valve not unheard of but rare.............
Good luck with getting it sorted, nice cars & lovely engines when well maintained but they dont tollerate lack of upkeep.
Cheers