Dear Ed Troy:
I don't have the PlutoSDR with me right now, so I cannot test it now, but a few things to check:
-Make sure that your Pluto can go down to 90MHz alright; it is only rated to operate on 325MHz - 3.8GHz, and not guaranteed to behave well on such frequencies (even when you did made the changes).
-Try lower sampling rate; like 2MS/s. PlutoSDR is known to drop samples at relatively high sampling rates (and yes, I do mean that 5MS/s is too high).
-Try lowering gain; FM signals can easily overload the frontend of such devices.
If I replace that PlutoSDR source block with an Osmocom source block, change the carrier frequency to my local station, and attach a different radio, it works. (Can't test on a PlutoSDR now, as I do not have one with me now).
Regards,