2006 PONTIAC MONTANA, 4T65-E

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Master O.H. kit, all solenoids, rear plaentary gear, 4th hub/shaft, accumulator pistons, manifold pressure switch, chain and reman converter.

P1811

Original Customer Complaint:
Vehicle came in with complaint of harsh upshifts and a P1811 code stored. After watching shift times of upwards of 800 milliseconds and seeing the EPC command maximum line pressure, an overhaul was sold.
Current Customer Complaint:
4-3 flare on part throttle kickdown then harsh 4th upshift. All shifts feel normal until after a 4-3 forced downshift. If you give the vehicle a lot of throttle (more than 75%) the 4-3 downshift feels normal and the following 3-4 feels fine.

This only happens at part throttle (less than 60%)
Work Completed This Visit:
Pulled pan and found some damage to new accumulator pistons (chipping and burnishing at nipple that fits into the spring) Replaced entire assy housing and all with known good used parts. No change.

R&Red trans and disassembled. No burned clutches, no seal ring problems found. We did find a little grit in the 4-3 manual down valve in the valve body. Cleaned it and polished the valve w/ 2000 grit sand paper. Checked the channel plate sleeve for ring seal wear and none was found.

Reassembled and test drove. Problem is still there.

I'm out of ideas and need some help.
Summary:
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