Thank you so much for this. It's now tested, built, and boxed, and so far it's working perfectly for my application!Here is a circuit for a positive peak detector I have used many times. You could substitute a FET op-amp like LF-356. U2-B acts as a buffer so the voltage on the peak capacitor isn't drained off quickly. Using a smaller cap lets the circuit ramp up faster.
At some point I'd like to figure out why my original circuit wasn't working, but in the meantime having a working circuit is terrific. Thanks again.