actually it happend to me this past weekend too, when i had a lot of load on the car

but i have tc and lsd as well.

 

Knock on wood..........

I have the TC and 3.76/LSD and have not experienced this issue yet...............

 

Knock on my wooden shifter! that my Gs400 has not held 4th but on occassion while in light cruise around 40-50 mph the 4th to 5th shift would have a slight delay.

I have both TC and 3.76 LSD! I say the best upgrades and running smoother than stock!

 

It happened a couple days ago, I got it up to 60mph before it shifted to 5th. weird.

 

In my opinion - I say its mostly the 3.76 rear end. I did the LSD first and I notice the the delay a lot more before the TC but after the TC was installed - the delay was less. Now that I have put some miles on both parts. Its "almost seamless" - but i can still tell just a slight hint of delay every once in awhile - I think with my car it depends on the speed of the vehicle and what rpm - but in WOT - Its flawless! I love my Car! but whatever the delay that happens once in a blue moon is nothing to worry about unless the 4th holds and not go into 5th - then I would worry..

 

My guess is it's some sort of computer related delay with lower gear ratios. Maybe the car expects what's installed from the factory.

I doubt it's an LSD related thing, I bet you could get the TRD LSD and install it into the stock differential and not have this issue.