I made an earlier post saying my GT was having a weird problem shifting from 2nd to 3rd, sometimes it felt like the gears weren't quite meshed; feels like a couple of teeth in the tranny knock together when pushing into 3rd. Kinda like one gear is still slowly rotating when I shift. No problems shifting up and down in any gear, including from 4th to 3rd.
This weekend, I finally finished some mods to the Cobra clone and took int out for a spin. I hadn't driven it for several months, just too busy. D@mned if the Cobra's T5 didn't do EXACTLY the same thing! The GT's T5 is just over 93K, but the Cobra's T5 (and engine) are just over 52k. Something weird is going on.
I can think of two things in common between the two. First, I'm just not shifting 3rd right, always possible. Second, the Dex/Merc I have in both T5's is doing something weird to 3rd. I had a Valvoline Dex/Merc in before, and I had gear whine in all gears. Now, I'm using a cheap O'Reiley's Dex/Merc. No gear whine, but 3rd seems to be an issue.
before my rebuild,it i had the same symptoms as you when it got really cold out.
less as it warmed up.
during the rebuild,found that the syncros were shot,and a shift fork lost its little plastic pad,allowing it to wear the fork.
right now i'm using royal purple with no problems.
Seriously, if you ever have any problems with your Tremec transmission, just call the guys at TTC. They have the best customer service on the planet. I've had a guy leave his office, and walk down to the assembly floor to look at something for me. They are probably going to recommend stepping up to Synchromesh, which is what they recommend on their street/strip transmissions. Amsoil sells a generic synchromesh for about $4/qt.