Dies when brake/clutch is released AND when the mower deck switch is pulled on (in this case even when the brake/clutch is latched). It dies as though I've turned the switch off. No chugging or overloading so I suspect electrical problem, likely the brake/cltch switch?? When the brake/clutch pedal is locked it runs fine. Ithere was no indication of anytrouble during three days of use. Went out the next morning and the problem started with first crank.

Any ideas of how to diagnose the problem without just parts changing?

Hi,
The culprit is the seat switch or the wiring associated with it. It is called "Operator Presence" in that the machine must "know" the seat is occupied by an operator before ir allows either of those events to occur.
Peace,
Clarke

Thanks, that was the issue. I had checked the plug on he switch prior to posting and it seemed well seated but upon closer inspection the fastening clip was broken and the plug was not fully secure... a little tape and all is well.