Climbing long grade; truck starts to shake and losses power

Ever since I've owned this truck sometimes on a long grade when I would downshift to 12, then reach the top and go back into 13th the truck would shake and have little power for the first few seconds, then it would go back to normal.

3 weeks ago I had the motor overhauled and now it seems to do it more often, and this morning it did it in 13th while climbing. Bad shake, loss of power. I was able to pull off on the ramp and let the truck cool down a bit, and it was fine for awhile after that.

So what would cause this? It's only happening when the truck is really working and getting warm, I'm thinking the Clutch? Getting warm and slipping?

Truck is an 06 387 with a 600hp C15, Eaton 13 speed. Any ideas?

 

Well, only an hp shortage when it starts shaking, otherwise since the rebuild the only thing that slows me down on the hills are idiot drivers who can't pull em

I'll watch the rpms, also going to see if I can angel the clutch next time it happens. I took a lighter load to hopefully get me out of her without issues though so we'll see what happens.

 

It did it before the rebuild as well, so my thought was it isn't engine related, I have a code scanner and nothing has shown up yet, other than a slight valve cover gasket.

As far as the shop goes, this is a nationwide Kenworth shop, and frankly one of the only places I would let touch my motor anymore. Todd's the man there.

I'm usually 78-80k. Normally don't have issues unless I'm in a bit nasty hill (can cross Pennsylvania at 80k without down shifting once.) Yesterday I was 78,500 and losing 15 mph once it started acting up in average hills.