If it is possible take the switch or router into the same room. Connect with a short cable.
If it not possible plug your PC into one end of the long cable and leave the other end in the router or switch.
Alternative connect the CONS port on the switch router and connect your PC at the other end of the cable and see what happens to show runn command. Look for corruption.
If OK its the long cable, the distance or EMI interference on route. I would go for EMI
There's LOTS of things that could be going on from a cabling perspective.
When doing cable installation, simply mapping out the pairs is nice (and I'd say a good first step to avoid most lunatic fringe situations) but it doesn't do anything to the TRUE capabilities or certification of the cable itself.
A REAL cable tester is needed for that. For 100M, you could even find any of the old Cat5 testers (EBay's good if you don't own one already and don't want to purchase a new one). They'll still test Cat6 cable just fine, you just won't be able to test to the full potential capacity of the cabling!
But baseline certification for your requirements is paramount!
I believe the 3750 switches have built in TDR (Time Domain Reflectometer), which could provide some insight into what's going on. Check out the url http://www.cisco.com/public/news_training/itsnews/tech/readertips/200802.html