Why are you using a floating holder with Rigid tapping? This is not needed and I have found it can be a detriment. In addition you cannot repeat rigid tap (peck tap) with a floating holder.
Keep your coolant richer than most manufacturers recommend; we use up to 15% and can expect problems when it gets below 10%.
How low is low rpm? I have found that sometimes the rpm can be too low, the chips seem to flow better at 1000 rpm than, for instance, 500rpm. On taps smaller than 1/4" go above 1000rpm and if necessary use a largish R for acceleration space.
If possible tweak the thread percentage; are you allowed to go down to 65% as opposed to 75%? This can help immensely.
Use the correct tap; machine taps, spiral flute for blind holes, spiral point for through holes.
Repeat rigid tap, aka Peck Tap, especially when going deeper than about 1-1/2 x dia.
Consider form taps, I don't use them so someone else will have to comment on this.