It can be radio interference. ... most packaged radio equipment despite their marketing claims in a usual urban neighborhood is good for 50-100 ft of distance before succumbing to rf noise and glitches.
Try to drive it in an open field or area and see if it does the same thing.
And if your manual does say 7-cells, then I guess it can handle more power. Though be warned that running more volts will heat the motor up more, and I don't know how hot your motor runs right now for the few minutes you use it. A very broad but general rule of thumb is if you can keep your finger on the motor for about 5 seconds, it is not too hot. If you allow your motor to reach 200' F, it will ruin the magnets inside it, and won't work any more.
Also, running more volts with identical battery capacity, e.g. 6-cell 1800 mah vs. a 7-cell 1800 mah, will result in lesser run times, because a motor will pull more amps with higher voltage being fed to it (more amps being drawn from the battery means lesser runtime). "Ohm's" rules of electricity... blah blah blah
Though again, I'd stick with good 6-cell packs and maybe one or two 7-cell pack(s) for the occasional speed itch.