reprap i3 z carriage motor trouble

So I have a perplexing problem that I hope someone can shed some light on.
For the past 2 weeks every time I printed I had to manually level the bed and each time set the z offset higher and higher. It was annoying and didn't think much about it though as I like tinkering. fast forward to a few days ago when it finished a print there was a grinding noise and the z carriage tried go up at a slant.
OK well this has happened before 1 or two times so I home it and realign everything. use my callipers and measure the height to get it perfect. AWESOME solved my problem! of the weird change of bed levelling every time. next prints, prints great. then I measure the z axis again and the left side is not as high as the right. re align and print. does the same thing again and again. its off by maybe half a mm or so everytime
when it finishes printing I have it move up to 160mm and I monitored it/touch the motor and I could feel it skipping steps every once and a while.
I have lead screw, I have oiled them. I have manually moved them and they have very little resistance.
I have not changed any settings or firmware in months. I have checked all cables to both motors are intact and connected fully.

The one thing my printer has never been able to do is print at full height. they company said it could do 190mm but it only goes up to 170 then starts skipping steps and fails hard. however, I have never had it skip steps for months up to 170 so I never print anything higher than that

what could be wrong? the motor? voltage drop to the farther one?
my printer is hictop prusa i3 aluminium version with mks1.4 board with upgraded leadscrew. and I was going to actually upgrade the PSU, hotend , heated bed and fans to 24v as supposedly it would be less wine in the motors and more efficient at heating up. Would this potentially solve or help this problem I'm having

Edited 1 time(s). Last edit at 09/28/2016 07:30AM by bizit524.

anybody? any suggestions?
There is one thing I have done and it seems to have improve the situation slightly.
What I did was loosen the screws that mount the motor to the frame so the motor can move side to side if it needs to while still staying in place. this hasn’t resolved it completely but it skips less steps now

Offered only in the remote possibility that it may be your problem. Is it possible that the bed leveling screws/nuts are unwinding during operation? I had this problem on a Prusa Mendel and had to replace the nuts with stiffer nylon ones.
Mike

I will try this, I am going to go ahead and upgrade to the 24v system this weekend and well replace the springs with ones I have ripped out of a CPU heatsink they are much much stronger
However how would that affect the z axis sipping steps?

Well I have upgraded to the 24v system. very nice. heats up quicker and seems a little more responsive? stronger motors? idk. I printed a few prints and it doesn't seem to be off any more but to be fair I didnt monitor it much. just when it finished I homed it and then measured. maybe taking half of it apart helped realign something that I was missing. will continue to monitor then update

Any chance of pictures of your machine?
Make sure the Z lead screws are not constrained at the top.
Check the Z guide rods are parallel.
Do you have separate drivers for the motors or are they wired in series or in parallel.

How are your Z-axis motors connected? I used to use tubing attached with zip ties until the threaded rod unscrewed itself out of the tubing. I use solid couplers now.