Define "a few minutes."

Also, do you still maintain control of the airplane, or does it completely shut down?

The new, hotter motors will draw more current from the system and drain the batteries down faster. Full throttle flight times will probably be cut roughly in half. Maybe you're seeing the normal low-voltage cutoff?

What kind of batteries are these? 2300mAh is an odd size for a NiCd, so I assume they're not brand-name cells like Sanyos, which are typically 2400mAh. It may be that these are cells are of dubious quality, and cannot handle the current you're asking from them. As the current draw on a battery goes up, the voltage drops. A good battery will hold its voltage, but cheap off-brand cells will often curl up and die under any serious stress.

That Jeti speed control is rated at 35 Amps continuous, right? I can only find reference to a Jeti 35, which is also rated at 35 Amps. Hobby Lobby's site claims 14 Amps per motor with 4.7x4.7 props on 7.2 Volts. It's entirely plausible that with more cells and/or larger props, these motors are in the 18+ Amps per motor range, borderline for that speed control.

Try a Great Planes 50A speed control or something similar.