I had the same problem in my 88 gt, only with 3rd gear and it turned out to be a bad syncro. I had just had the tranny rebuilt and took it back and they fixed the problem, but like rev said, it may be as simple as a bad clutch adjustment.
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James Cox
nochevy@hotmail.com
1991 Mustang LX
Best 1/8 time on motor 7.686 @90.20mph