Belts jump after lawsy carriage mod

An easy fix and huge upgrade would be to switch to fishing line.  Have a search

Not sure what i need for fishing line??

In any case the rods seem to run smooth when doing the tensioning. And without load the y-motor pulley tends to feel a bump at about every 50mm like if the belts were skipping a tooth.

Ps. Wouldn't fishing line have streching and slipping issues??

Every 50mm is approximately every turn of the pulley - check they really are free and running true, and the belt isn't rubbing on the flange too much.

Belts should be 0.08" (2.032mm) pitch MXL, 1/4" (6.35mm) wide for the stock solidoodle pulleys.

This is a really hard one to work out without being there - any chance you could video the thing skipping around, close ups of all the pulleys, carriages, whole overview, and upload it to youtube? Big ask I know, but otherwise the best we can do is stab in the dark. It would also help others perhaps trying to diagnose their own issues, being able to compare to yours.

I never would have thought...

You have the videos marked as private

OK so the drive belt pulleys to the motor should be moved as close to the motor as possible.  I would even flip the one on the motor to move the belt even closer.  That belt is plenty (too) loose but is perfect for what you are trying to fix just note that it will have to be tightened. 
What belt is skipping (left, right or drive) and roughly at what position front to back when it skips?

+1
That small drive belt seems very loose at around the 0:38 second mark in the video...

Don't waste money on an SD replacement.  You can get much better motors for less a lot quicker.  Ask for links but that is not your problem so lets fix that first.

Many of us want to help but let's get on the same page.  First things first, flip that pulley on the motor and get that belt closer to the motor.  This will create a much shorter lever arm and help prevent deflection.  Then tighten that belt like we suggested and report back. 
The distraction of a bigger/better motor will not change the fact that your belt is skipping.

I can assure you that we will get your problems solved.  Put that printer in the hands of any of the experienced users that have replied and they would have that thing printing perfectly in less than 20 minutes.
What I am saying is help us help you by communicating clearly and staying on task.

Did you flip the pulley and tighten the belt?

Wow wardjr you said 10 min to fix? With your suggestions I got it under 9, Adjusted the the direction I re-tensioned , All seemed to work, I'm running a new fan mount to test. Thanks for the info, I'll let you know the results when done.