Weird ESC Problems

[Major Azzwoopin]

I got a Associated RC18MT a few weeks back. I have the mamba 5400 kit but haven't installed it yet. Well I was running the stock motor and ESC on two different battery configs. One was the stock 1100 mah 7.2 volt pack, the other was a maxamps LI PO pack of 2s2p 900mah cells.

The thing ran alot better on the LIPO packs, but the other day the ESC stopped "recognizing" the 8.4 volt lipo pack. When I connect it to the esc input and turn the switch on it does nothing. No lights nothing at all. I measured the voltage and it was at 8.4 I have let it drop and it is now at 8.2 and still nothing when connected to the LIPO pack. If I hook up a fully chared nimh 7.2 pack to the ESC it works fine. A fully charged venom 7.2 stick pack was reading in at 8.05v and it worked just great also...
Any ideas... Is that extra .15 of a volt beyond the tolerances of the ESC.. The ESc says it will work up to 8.5v??

I am lost..thinkin about just puttin the mamba in.
Only reason I haven't is my 2 and a half year old is having a blast with the rc18 with the stock motor and I wanted him to get a little more comfortable with the stock motor before I overwhelmed him with the mamba power. I'll have to post a vid when I get the chance. You won't beleive it is him running the thing. He runs it and my rottweiler chases it all over the place. I am amazed at his ability to control and get around obstacles and drive it right to where you want or ask him to.