there are a number of things you can try.
shut the high pressure water to pre fill, stop rotation give the drill a few minutes to release pressure, try raising the drill stem if the drill stem raises or lowers return the HPW to full cutting pressure and raise the stem until you are out of the coke bed. this works most time on sponge coke
if you have shot coke;try using this same tech except try to lower the drill to the bottom of the drum, try and keep the number of passes thru the hot areas to as few as possible, the cold water will cause more eruptions
If you need any other suggestings I cut coke for over 25 yrs