y axis issues

help, going nuts!!!!!

as soon as a load is put on the y axis, it stops responding. if i run a small g code routine "mid-air" consisting of 2 x 6" lines with 4 passes each line on continual mode (code "99"), it will generally stop moving on the y after about 10 to 15 minutes.
spindle keeps turning, x and z continue to work correctly.
the display (mach 4) shows the g code running correctly, even for the y axis which is not moving.
changed the power supply yesterday, tested with a small g code routine (3 squares with tabs, 4 passes per square), ran fine for over an hour (not touching a surface--mid air).