4jx1 missing

Hi all.
i bought my jackeroo about 6 months ago, and until last week i could not fault it. On a short trip to the shop to buy some lunch it suddenly started running very rough, like it was missing. At idle the whole car shakes, but doesn't cut out, and no problems starting. Big loss in power.
Symptoms are the same whether hot or cold.
i changed the fuel filter which seemed to improve (but not fix) the problem. However that was very short lived. I cleaned the wiring loom connection near the dipstick (was full of oil), but this had no effect.
i have removed the injectors, all 4 were covered in oil all the way down to the tip. The tips had a lot of hardened black soot on them as well. Can anyone give any indication whether this is a injector/injector seal/ sleeve seal/ or maybe orps problem i am having? The oil level hasn't changed, neither has the coolant, but the coolant has now changed to a browny colour. Would this indicate their is diesel getting into the radiator, or is it just getting old and needing replacing?