cutting problems

Hi i am running a prt 96 with cvarve pro and Mach3
been cutting a number of letters this morning after a few min cutting i heard a thump after the job was completed i then notice allot of the letters did not cut through i then ran a few more files and had the same problem. also notice occasionally i get a rumbling sound coming from the machine. when the maching is not cutting just sitting I thought it might be the bob PMDX -122
i disconnect the supply to the z axis from the breakout board.
and still occasionally get the sound. any idea where i should start looking?

thanks Paul