Power Drop on repowered vintage Losi XXX-T

[OneSocko921]

Hey There. i recently repowered my old Losi XXX-T sport with a 12T Traxxas Titan and a hobbywing quicrun 1060. geared at 20/84. Running off 5200 2s 50C Lipo. it's running great but i am getting a big power (speed) drop about 5 to 10 minutes into bashing. From my understanding, Lipos should run full out until the safety on the ESC shuts the car down. I turn the car off and on and i get full power for a couple more minutes, then immediate drop in power. i want to take this thing to my local track to rip but i want to fix this issue first.
Any ideas? My thought is the motor is overheating but I'm not positive.

I'm not looking to race at the track. just run it for fun during open practice.