Hi all,
Need some help determining how to diagnose some codes I am pulling. It has been going on for a couple of years now. Intermittent Check Engine codes P0137 and P0131.

1996 Volvo 850 Turbo wagon
VIN: YV1LW5720T2247599
Engine Type: 57 - 2.3L 5 cylinder
Front wheel drive
Mileage: 205 K

Pulled this info from a code reader:

Code P0137 - O2 Sensor - Circuit Low Voltage
Bank 1 Sensor 2

Code P0131 - O2 Sensor - Circuit Low Voltage
Bank 1 Sensor 1

Freeze frame from code reader I pulled the next day. Not sure what all this means if anything:
DTCFRZF P0131
Fuel System 1 OL
Fuel System 2 N/A
Load_Percentage 2.0
ECT °C 82
SHRTFT 1 (%)
Long FT 1 (%) 0.0
RPM (1 min) 800
VSS (Km/h 18

Long term symptoms:
I seem to be getting the CEL after driving around 100-120 miles mostly on trips but eventually when I reach this milestone to set the codes . Most of the milage is highway driving 55-65 mph. Sometimes comes on when reducing speed going through small towns or slowing for stop signs or variations of speed on highway. Also will hit the CEL after driving about that mileage over time with variation of mostly highway and in town driving.

I need some info on how to attack this. Is it O2 sensors; relays; electronics going to injectors; fuel pump; fuel pump rag filter; computer or some other malfunction? Can't keep throwing money at it to find a solution without an educated guess on where the problem/s are.

Here is what I have done so far:
1. New plug wires and plugs
2. New distributor cap and rotor
3. New Bosch upstream O2 sensor
4. Cleaned all injectors - Pulled out of fuel rail. Made tool to spray carb cleaner through injectors when injector is opened with electrical charge. All injectors worked good when sprayed carb cleaner through them as supposed to do.
5. New injector o-rings top and bottom. Did not replace fuel rail o-rings for injectors or seals on intake manifold. All seemed fine.
6. Brand new inline Gas Filter
7. Brand new Air Filter
8. Not sure of the cat but no exhaust leaks or problems that I can determine.

I am at a loss as to where to go with this. Why would it take so long to trip the codes?

Engine runs great and turbo working as it should. Getting 24-25 mpg on mostly highway driving.

Anyone? Let me know if you need more info on something.

Thanks for your help in advance,
Clark