Synchronizers are components contained in a manual transmission that allow for easier shifts between gears. Prior to their invention and application in car transmissions (long ago, at the dawn of the automotive age), it was necessary to match the speed of the spinning flywheel to the unsynchronized gears in the transmission (though usually those early engines were huge, with great torque, and there were often few gears involved).
When sychronizers start to wear out shifting becomes difficult and "crunchy", particularly when the relative speeds between the rpms of the moving masses represented by the spinning flywheel and the drivetrain downstream from the transmission are not nearly the same (the closer these rpms are, the easier the shift and the less the burden on the synchronizer).
The sychro's in most Cobras (and other Mustangs) before they went to the stronger 6 speeds are a known issue, particularly in high mileage cars. These transmissions (which include the T5, T45, 3650, etc, and various iterations of each) were designed for the STOCK power output of the cars, and since most Mustangs end up being modified to deliver a lot MORE power, and are driven hard with multiple mishaps during gear changes (at least partly due to the rubbery, loose OE shifter)...
Well, the logical problems follow.
For this reason several companies have come up with stronger, upgraded replacements that will last longer and tolerate higher power levels. Installing them requires a good bit of knowledge and some special tools, and the parts are not cheap (particularly the desireable top quality upgrades). Unless you are an experienced mechanic, I would leave this work to a good transmission shop.
__________________ tripleblack
"You can never be free until you let yourself go." | 
