i just sent my t5 to a guy in pa who rebuilds mustang trannies (manuals).all he does is rebuild those type trannies he really knows his **** . my trans was rebuilt by prevois owner but wasn't done right.it took this guy 2 minuets to tell me all about my trans and what was probly done wrong during last rebuild.go to this guys web site check it out
www.hanlonmotorsports.com . if you want to still go with your shop (ie because they have your money)then tell them that you want them to cusult with this guy to find out what they are doing wrong..my trans was doing similer things guy said that input shaft might not have been shimed right.if you can get refund then send trans to him or bye one of his rebuilds .....he also deals with tremecs could set you up with one of them........good luck
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old guy with 87 gt....well not so old guy