Need help with ESC problem!

[jsr]

I'll start off with my setup:

Kyosho DBX8e (a nitro DBX I converted to brushless and 1/8-scale)
- weighs just over 5lbs with battery
Hobbywing EZRun 60A ESC with external UBEC
Medusa 3300kV 3650 motor
3s LiPo

My buggy's been acting strange recently. My old 3s packs (3000mAh 30C) are getting old and I could feel them losing punch at my last track outing. So I picked up a couple of new Turnigy 5800mAh 25C 3s packs. When I tried them out this past weekend, the buggy accelerates hard (harder than with the old packs). As it ramps up in speed, it feels like it's pulling really hard. As it approaches top speed, it slows down a bit and tops out early. Using random speed numbers for example sake, here's what's happening...

- Accelerating at full throttle, it pulling hard (say at approx. 35mph at this point) and seems like it will pull to 40mph
- As it hits approx. 35mph, it slows to about 30mph and stays there

I tried both old and new packs and it does it with both. As I ran it more with the old packs, it would do it less often and sometimes hit full speed. As the packs ran lower in voltage, it started to run normal and hit full speed. Because the new packs hold voltage so much better, it would always cut-back and slow down. I checked wiring and nothing is cut or damaged.
If I slooowly roll into the throttle, I can get it to hit full speed (which is faster with the new packs than with the old). But if I get on the throttle just a tad faster, it will cut-back.

When it does this, both the ESC and motor are running hotter than normal, about 20-30F more on the ESC, only about 10F more oh the motor.

I checked the drivetrain for binding and all is smooth. I recalibrated the ESC and Radio. I confirmed (by holding the buggy) that the ESC is seeing full throttle (via an LED on the ESC that lights when I hit the max throttle point).

It seems like it's a voltage related problem with the ESC, but why and what? What would cause it to slow down due to higher voltage (or run to full throttle with lower voltage)?
Need help as this is driving me nuts and made me cancel my last trip to the track.

Thanks for the help.