"It starts and runs perfectly until it warms up, ambient temp about 65 this means 3 minutes, then it completly lacks power and has a surging idle ( almost stalls then recovers)."
The surging idle when warm and almost stalling went away when I replaced the ignition amplifier. No codes were set.
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1980 245 Canadian B21A with SU carb, M46 trans, 3:31 dif, in Brampton, Ont.

Since you just replace the O2 sensor, based on your response, we are unlike to convince you to do the multimeter test. This leaves engine temp sensor and fuel pressure regulator as the likely suspects.

Any chance the exhaust is obstructed? Maybe pulling the battery cable for 20 minutes and letting the ECU reset its fuel maps would help.

If you accelerate slowly, will it climb rpms, or does it hit a wall at some point? My money is, as usual, on fuel/mixture.

Like trichard says about no codes. Have you checked the OBD (on-board diagnostic)?

From the Volvo 700-900 FAQ, hosted right here on your favorite brickboard:

https://www.brickboard.com/FAQ/700-900/EngineOBDCodes.htm

Intermittent causes may not display at first as how Bosch set up the OBD as meant to be forgiving and adjust to aging sensors and such, then set a check engine light.

Learn about some causes for poor performance symptoms ...

https://www.brickboard.com/FAQ/700-900/EnginePerformanceSymptoms.htm

You'd need a mulitmemter with resistance (Ohms) check and continuity function, in addition to DC and AC volts check ....

https://www.brickboard.com/FAQ/700-900/EngineSensors.htm

Do you have your Bentley Volvo 240 Service manual, or at least a Haynes Volvo 240 service guide?

Could be:
- Starting to fail throttle position sensor / switch.
- You do not mention a gasoline smell, yet a leaking fuel pressure regulator (FPR).
- Connection to the engine control coolant sensor is bad, while the connection is good, the sensor itself is failing as aforementioned.
- Clogging catalytic converter.
- A fuel delivery problem could be a faulty 25 Amp fuel injection system fuse in the 1990 (final year for this, uh, well-known feature) 240 engine compartment, on the North American driver side, on the small metal horizontal rail in front of the inner fender / strut tower, or other faulty fuse, or other electrical problem like the fuel injection system relay (cranked solder welds).
- Hot air coming through the pre-heater hot air intake through the air filter box. The flap valve in the air filter box gets stuck to hot air open. Hot air coming through the silver accordion hose that connects between the exhaust manifold heat shield, under the water pump fan shroud, and into the bottom of the air filter box. Hot air on the AMM on a warmed-up 1990 240 with Bosch LH-Jet / EZK may cause running problems. Immediately remove the pre-heater pipe connections! Else you'll ruin your new AMM/MAF!
- You may also have air intake (vacuum) leaks. Inspect and diagnose the air intake, including all vacuum hoses.
- You may have an exhaust system leak at any point upstream of the 02 sensor. Check the cylinder head to exhaust manifold connection. (Soapy detergent water in a spray bottom on cold exhaust pieces just as you start the car. See bubbles?) Sometimes the three securing hardware sets securing the header pipe to the exhaust manifold get loose. Or the securing hardware at the header pipe output to catalytic converter input (a particularly prone to fail exhaust union on RWD Volvos - most people to not ensure the header pipe is secured at the downstream end on the factory bracket).

There may be other causes. Hope this helps.

Other will chime in to help you.

Questions?

Dud MacDuffed.

using a meter i would revisit the o2 sensor and verify it swings rapidly within range once its warmed up.

also check your coolant temp sensor.

it is odd no codes have been set

My recent problem was the ignition.
Ignition wire from coil to distributor cap.
Cap and rotor.
Spark plug wires.
--
1992 Volvo 240 & 1998 Volvo C70