Did you have another crew who can fly,competently..?
Put your wingman on side away from river..did he notice any flames/black smoke when it lost power..? no signs of fuel leak/oil leak..?..how is the cyl .hd temp.,steady ,or up/down abnormal..?.
After you turned about,I would have set about climbing,even 100-200 f/min,provided I was not reducing speed too much,as every little helps...
It seems like an intermittent fuel flow problem,or possibly a choked intake,possibly ice,or dust,so change it to hot/alternate.
I`d also set the `primer` to CYL if it`s like a Yak,as you may have fuel vapourisation...you may also have a blocked fuel vent....
Otherwise ,climb as much as you can,and if it quits,hit the primer to keep going. If it don`t work,head for the beach,assumin 7700 is on,get the PROP into minimum RPM,cooling gills open,keep the a/c clean,and crash `under control....
Still here,you did a full fuel drain check before t/o....?