When it hit 74mph did it run ok through that entire pack? Did you run it after that? I mean, how many packs did it run through hitting those kind of speeds?
If that was the first and only time it hit those speeds, I would suspect you might have overheated the ESC, motor, or wiring. If it ran good a few times after that before your battery died, It could just be a bind somewhere in the drivetrain. But having that battery go bad, the recent gear change, and the thermal shutdowns it sounds more like you have some electronic problems.
Regardless of what it might be, I would start tearing it down. Start with removing the pinion. Check the drivetrain by rotating the spur to see if it feel like it's in a bind. For it to slow the car down that much, if it was a drivetrain issue, you would definitely feel some resistance trying to rotate the spur. If it feels smooth, I wouldn't get in an immediate hurry to check the motor or ESC. You could have simply melted your battery connector a bit, or weakened a solder joint due to that battery going bad and being tasked to deliver more amperage to the ESC than it can handle. So check your cables real quick.
