Weak/choking engine
 

mostly stock 1.6 with brain-built MS3X + 640cc FF injectors
I have had no real problems with tuning or the car in general until now. I filled the tank with 91 for the first time last week and noticed I had somewhat rich afr's, but nothing autotune wouldn't take care of.
So I drove around for about 2 hours yesterday and at the end of my drive, I go to fill up. The car was running like an absolute champ the whole time and my afr's were exactly where I wanted them to be

When I drove through town, my afr would flick rich every few seconds. Its happened before and usually goes away with time or a restart.
As I'm leaving the gas station, the whole car suddenly decided to run like hot garbage; it was stumbling the whole way home running 9-11 afrs if I went over about 50 kpa and if I gave it anything over that, it was like the engine was being choked out and protested anything over 2200 rpm
There's also a really weird sucking noise coming from my catalytic converter, of all places. I've had an exhaust leak around it for some time, but it went from normal exhaust leak to sounding like a light sneeze every time I let off the gas and the end of a vacuum cleaner when I'm on the gas.

I tried restoring a previous tune and it didn't help, so I believe it's a mechanical problem or a really bad tank of gas. I'm getting spark and all injectors are firing properly.
Has anybody else had this problem or can point me in the right direction?