***** main problem: with what I think is a fully charged battery the tank starts up and drives fine for a few minutes then everything stops working
except the sound which just sits at idle??? also smoke works. ******

It happened to my Tiger 1 as well at the first few weeks out of the box. Also when upgrade the Pershing to Li-Po battery came across the same problem.

[link]http://www.rcuniverse.com/forum/m_6256756/anchors_6256756/mpage_1/key_li%252Dpo/anchor/tm.htm#6256756[/link]

The wire that connects the antenna and receiver board is usually badly oxidized and filters out the RF signals. Replace this piece crap of wire with a new one or change to Pz's easy antenna mod.

The broken capacitor across the motor brush also the cause of interference and make the tank stall. Get a replacement and solder it back.

Hope this help.