Surge above 40mph

Guys I'm out in Moab and could use an assist.

Problem:
Engine surges by 250-300 RPM at any speed above 40 and only when cruising in Overdrive (AW4). If I accelerate and get into passing gear the rpm's climb and hold steady. As soon as the rpm's drop back down it starts happening again.

FYI...No Check Engine Light

What I've tried:
Replaced the IAC...no change.
Replaced the TPS...no change

Crank and Cam Sensors are both only a few months old.
Fuel pump is about a month old.

No problems crawling around on the rocks, just on the road in OD.

Anyone got a cure?

40 MPH at operating temp is the sweet spot for computer to tell the IAC to open fully.

Does it happen when engine is cold?

I don't recall to be honest.

Bump for some new insight.

Sounds like the torque converter is unlocking. Mine does that intermittantly too. I still haven't figured out why.

Wow...back from the dead!!

What's happened since this was posted.? Hmmmm....
On the way home my motor crapped out 10 miles north of Moab. Cam lobe failure.
Rebuilt and wheeled throughout the summer and still had the occasional surging issue.
Lost a valve spring and that caused another partial rebuild. Still surging occasionally.

So I'm now 100% certain that it is not a mechanical issue related directly to the engine.

Torque converter locking and unlocking? Your guess is as good as mine.

This is what the FSM says abut PCM input at Cruising Speed:

CRUISE MODE​

When the engine is at operating temperature, this
is a Closed Loop mode. At cruising speed, the powertrain
control module (PCM) receives inputs from:​

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by the A/C thermostat.

Frank Z said:

Torque converter locking and unlocking? Your guess is as good as mine.

Click to expand...

That's my guess too. The clues are that it only happens when you are in overdrive, and isn't the RPM differential that you are experiencing similar to the normal difference when it drops into overdrive?

Honestly, I'm not too sure.

wouldn't hurt to check the wiring circuit for the brake light/ cruise control switch. I know the converter will unlock with slight brake pedal application, so a loose connection or faulty wire could cause yer problems.