I assume this is the brushless version? Nimh batteries (especially such small ones as 1800mAh) are not adequate to drive brushless motors. I expect the problem is that the ESC has a low voltage cutoff for lipo batteries? If that's the case, it is likely to also be the problem. You see, when you employ a lipo battery, it is critical that it doesn't discharge beyond a certain point, so we program that point into the ESCs (manufacturers do as well), it is usually 3v per cell for example. Since the ESC on its own can't understand if it has a lipo or a nimh battery plugged in, it treats the nimh battery the same way. This is a problem because nimh batteries will drop below 6v under heavy load, especially with the weaker smaller batteries, and the ESC is cutting off before the battery is depleted.

Either that or the ESC or possibly the battery itself, has a serious problem.

For what its worth, you really should take the plunge for a lipo setup. It's light years ahead of nimh. More runtime, more power, more speed, everything, just a little pricey in the first instance, but its stuff you only buy once at least.