So, when I am cutting with a tool at or longer than 1.5 inches and a harmonic or chatter sets up in the tool my Z starts to drop. the worse the chatter the faster it drops. its almost as if the vibration is overriding the holding torque of the stepper motor. this has ruined a fue of my projects so far and id really like to get this solved. has anyone else had this problem?
Welcome!
Are you sure it’s the stepper and not the bit being pulled out of the collet? I’ve found I need to clean new router bits or the forces from plain cutting will eventually pull them out.
In my experience long bits do tend to ‘scream’ more and that vibration can lead to worse cuts and/or the bit working out of the collect as djdelorie suggested. Two suggestions - runs slower/take small bites and see if that helps. Also, collets do wear and should be replaced from time to time. If you have a spare collet try it and see if things get better. I just happen to replace my 1/4" collet this morning and the cuts today have been significantly quieter even though I’m not using a particularly long bit. I switched out the collet after a bit slipped in the old one and ruined a work piece.
Another suggestion - measure how much bit you have sticking out of the collet and then if the problem happens again measure again. Then you’ll know if the bit is slipping or if there is a problem with the Z axis.
Also try faster/bigger bites. Most people aren’t moving their CNC machines fast enough to optimize the chip load on router bits.
Of course, there’s a fine line between “fast enough to cut efficiently” and “fast enough to shake the machine apart” 
100% sure it’s not the endmill sliding out. i even marked the body of the tool with a sharpie to see if it was coming out of the collet. Also, i should have probably mentioned before but i program and run cnc for a living so i know a bad collet when i see one and i understand feeds speeds and radial chip thinning, i should have been more forthcoming with that. Sorry, i feel like that sounds a bit pretentious but im just trying to convey that i understand what you are saying and i have taken that into account 
. i clean my collets, spindle and nut very well every tool change and i make them as tight as i dare. the wrenches supplied with the machine tend to bend open pretty easily so i try not to go that far so not to damage the collet or the tools. Any other suggestions?
Long bits scream pretty loud on mine in hard wood as well. Especially mesquite. However, I never lost Z (unless my bit slipped), but mine is a straight bit.
Are you using an upcut bit? I would think if you are losing steps down, it would have to be an upcut bit pulling down on the spindle. I have never heard of anyone having this problem before though.
Ok, if it’s not the bit slipping then are you using Mach4 and stepper motors? I had a situation where if I was running too fast with 3D toolpaths, especially in the z direction, I would hear an audible clunk and the z axis would be way out. In my case I the the problem was a barely in spec computer not pushing out the gcode fast enough such that I lost steps and generally weird stuff happened. I upgraded the PC and the problem went away. Not saying it’s what is going on here, but maybe.
jjneeb its a brand-new diablo 1/2" straight cut bit. GJ i have the nema 34 mach 4 but this happens in a straight line cut and only when the chatter gets really bad. i think ill take a peek under the skirt of the z. maybe the spider that drives the z screw is loose enough that it can spin the ball screw but not the motor? idk.
Ya, thats a good idea. I have heard of that coupler coming loose before.
You might want to try a spiral or compression bit to reduce vibrations. Straight cutting bits can create more vibrations and harmonics. You might also try putting some sort of flag on the top of the z axes stepper shaft to see if it turns while making some long test cuts.