Its not likely to be mach3 per se, but a combination of factors. What voltage is your power supply, and what ist the max current it can supply? Have you been using these picsteps for a while, or is this early testing? were the lmd's running quite hot before you changed the values? Do you have heatsinks on the lmds?
IIRC lmd18245 is rated 55v 3A max. That is the absolute most you can get out of them without risking destroying them, and near that power level you should be VERY careful to make sure they are properly cooled, and the power supply voltage never exceeds the rating.