turns out it was battery swell.
wiped the phone, reloaded from backup - no change. [br]
upgraded to latest IOS and loaded apps from fresh. seemed okay for a couple of days. [br]
as soon as i put the case back on it started up again.[br]
as i was removing the case, i saw some back-light leakage on the side, between the screen and back case, about a 3cm long gap. [br]
the battery was 4.85mm thick at the widest point (a normal, new battery is 3.25mm)
thanks to all for the input and advice