lxsSC400

So I've been trying to diagnose my SC400 for the last couple days and am looking for some guidance.
The car starts up and runs perfectly for about a minute. After that, it runs very rough at lower rpm (<2500). When you put the car in Drive with the brake holding the car in place, the whole thing shakes and seems to miss firing a cylinder. The rumble happens at ~40 mph (~2000 rpm) unless I turn off overdrive.
This is my second SC. When the first was doing something similar, it was rough at all speeds and rpm. That was a simple ignition coil replacement for the driver's side.
I thought this was a similar situation so I replaced the passenger side coil and all the spark plugs (driver's side coil is confirmed good).
The car has no CEL and gave no OBD 1 codes (just endless CEL blinking)

I checked the resistance of the driver's side fuel injectors and the frontmost passenger's side and they all are within spec. The others are under the Throttle body so I will test them if I can eliminate all the components that don't require the engine apart.
I used a jumper to bypass the Fuel Pump ECU and that did nothing
I tested and got sparks on all 8 cylinders

The O2 sensor 2 was working correctly according to the voltage feedback at the diagnostics port but O2 1 may need to be replaced.
The other part that I suspect from reading other posts is the ISCV

Any thoughts?

lxsSC400

Update:

I pulled the ISCV and measured resistance at the terminals as per the service manual. B1 to S1/S3 gave 46 ohms and B2 to S2/S4 gave 45 ohms while spec is 10-30 ohms. Looking into replacement options (over $1k new at Autozone)

t2d2

iTrader: (8)

Look for a used one. They don't go bad often enough to warrant that kind of dough. Max it'll cost you is $50, and probably half that.