Arc voltage randomly changing

had this problem once before and went away for a while but popped up again yesterday. i cut about 7 sheets of 14 ga material with voltage set around 145 per machine spec. cut all 7 sheets flawlessly. load sheet 8 up hit run, sheet pierces and cut takes off and torch instantly starts to raise to the point it looses arc and shuts off. i thought maybe bad consumables so i change and restart program but doing same thing. i notice that the arc voltage is reading around 75 while cutting. its late so i shut down and come back out next day. same consumables with voltage set at 145 and cuts sheet flawless. load another sheet when complete and starts reading around 75 again and raising the torch. if i switch to smart voltage it will cut fine but is reading around the 75 mark on the arc voltage. any ideas would be greatly appreciated.

Don’t know what machine you have but my setting for Hypertherm is 128 voltage for 14 gauge?

i have powermax 1250. Per manual with 40 amp consumables on 14ga steel it states 144 arc voltage