I have a file located in "My Documents" named "Standard Parts". These parts are used on numerous assemblies. When i correct the placement of these parts and go into a different assembly with the same "Standard Parts", there are mate errors or/and they have moved.
Please see attached. The blue pins are from my "Standard Parts" and are floating in space. 
When you say you're correcting the placement of the parts, can you explain the process? If it's just a matter of adjusting Mates in the assembly, it shouldn't affect other assemblies using this Part. If you're editing the Part, then that could cause the behavior you described.
Im just re-adjusting the pin assembly to its correct location (im not editing the assembly).
Then the same pin assembly in a different trailer assembly is moved away from the position i left it before i saved and closed it.
If you're only adjusting mates in the higher-level assembly, but not editing the sub-assembly, then I can't guess how that's affecting other assemblies without seeing the files. Can you post one of the assemblies?
The pin is only used in completely different assemblies (not higher or lower level)
What does the pin assembly consist of? Is it multiple pins? If it is several pins that are floating in space, make sure you have the pin assembly set to flexible in all of the upper level assemblies.
This is the pin assembly
Is the pin assembly inserted directly into the different trailer assemblies, or is there another sub-assembly level in between? Actually, could you show a screenshot of the trees of both trailer assemblies? Or even better would be to pack and go them to a zip file and attach it here for us to look at closer.
How do you upload a zipped file?
When you reply click the "Use advanced editor" link in the top right area of the reply box. Then in the bottom right there will be a link to attach files.
I have attached the chassis of the trailer
Is the same chassis assembly used on multiple trailers? If you move the pins around within the chassis assembly they will stay in that same place everywhere that chassis is used, unless you leave them unconstrained in the chassis and make the chassis flexible in the trailer.
I would recommend putting the pins in the trailer assembly rather than in the chassis assembly. Then you have different instances of the pin assemblies in each trailer and changing locations/mates in one should have no effect on locations/mates in the other.
Your problem might go away if you mate the pin assembly or make it fixed.
Make sure there are not similar/same files in different location.
Is the same chassis assembly used on multiple trailers? If you move the pins around within the chassis assembly they will stay in that same place everywhere that chassis is used, unless you leave them unconstrained in the chassis and make the chassis flexible in the trailer.
I would recommend putting the pins in the trailer assembly rather than in the chassis assembly. Then you have different instances of the pin assemblies in each trailer and changing locations/mates in one should have no effect on locations/mates in the other.