2000 Chevy Impala transmission problem.

I am having a problem with my 2000 Chevy Impala LS transmission. On occasion the tranny will shift hard while driving. Always happens when the unit has been warmed up. It is not weather related. It will do it
on a cold as well as hot day. Out of the blue it will shift hard from 1st to second on up to overdrive. I can also hear a faint high pitched noise like a hydraulic pump when it does it.
This started about 3 months ago. I had the tranny fluid and filter changed 6 months ago at 70,000 miles. I had the tranny flushed and filled filled a week ago hoping to cure but it still does it. It now has 81,000 miles.
I took it to a transmission repair place while it was doing it and the tech plugged his computer/scope to it and we went for a drive. he could see the fault. The code indicated that the computer was puting the tranny into "high Pressure" Limp home mode. He said usually there are 2 codes present, one telling the computer something is wrong and the other the computer telling the transmision what to do. Mine only showed the code to put the trany in high pressure mode.
He was able to clear the code but it came back on a minute later. He said he would need it for a day or so to figure out what is going on and what needs to be fixed. My question to the group is will driving the car in this condition cause even larger problems to the tranny and does anyone have an idea what may be wrong. I'd hate to get the tranny rebuilt if the only problem might be a wheel speed sensor.
Any suggestions, ideas? Thanks in advance.
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This has been a somewhat common problem on these units, usually an internal press. loss occurs and to compensate the pcm boosts the line press. , finding the cause is another issue. What was the actual code? May be a problem with the bore in the valve body for the TCC.

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The code is a 1811C, also a P0714B ?, a converter stuck off code? I'm going to leave the car with them over night so it is cold and they are going to have a try at diagnosing it.
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Larry, these codes don't seem to be the same as you got, 1811 is for 4wd, 0714 is trans fluid temp circuit, this could cause the pcm to try and compensate for what it sees as an out of range trans temp reading.

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