I've got a 2000 Century that's been doing it for years.
I would have thought GM had fixed it by now.

Section: 07 - Transmission/Transaxle
Bulletin No.: 0O-07-30-002B
Date: July, 2002

Subject:
Slips, Harsh Upshift or Garage Shifts, Launch Shudders, Flares,
Erratic Shifts and Intermittent Concerns, DTC P1811 or P0748 Set
(Replace Pressure Control Solenoid Valve Assembly)

Models:
1997-1999 Buick Riviera
1997-2002 Buick Park Avenue
1998-2002 Buick LeSabre
1999-2002 Buick Regal
2000-2002 Buick Century
2002 Buick Rendezvous
1997-2001 Chevrolet Lumina
1997-2002 Chevrolet Monte Carlo
1999-2002 Chevrolet Venture
2000-2002 Chevrolet Impala
1997-1999 Oldsmobile Eighty Eight
1997-2002 Oldsmobile Silhouette
1998-2002 Oldsmobile Intrigue
2001-2002 Oldsmobile Aurora (3.5L)
1997-2002 Pontiac Bonneville, Grand Prix
1999-2002 Pontiac Transport/Montana
2001-2002 Pontiac Aztek
with Hydra-Matic 4T65-E (RPOs MN3, MN7, M15, M76)

Condition:
Some owners of the above vehicles with a HydraMatic 4T65-E transaxle
may comment on harsh upshifts or harsh garage shifts, soft shifts,
shudders on hard acceleration, or shifts erratic. These conditions may
appear intermittently or set a DTC P1811 or P0748. During diagnosis, a
low or high line pressure (actual versus desired) may be observed.

Cause:
The above condition may be due to any one of the following which may
affect line pressure output:

Sediment inside the pressure control (PC) solenoid valve, causing the
PC solenoid valve to mechanically bind.
Sediment in the valve body, causing the torque signal regulator valve
to stick.
Incorrect transaxle oil level.

http://www.automotiveforums.com/vbulletin/showthread.php?t=179354&page=1&pp=15

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