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Yes, as I'm rolling through campus going 10-20 mph, it feels like I've been rear ended when it shifts from 1-2. It seems like a fairly common thing with the 4L60E's in these cars, my friends '99 Trans Am does it as well. As long as it's not slipping or hesitating, everything should be fine with it.
Some faults will make it shift hard & early, and may
not set a code if intermittent enough. The shift kit
usually firms up shifts and maybe this was excessive.
Especially if somebody also upped shift pressures in
the "street tune".
If you can look at force motor current, see if it is
bottoming to 90mA or 0mA (commanded full pressure
during shift). That would implicate a fault if it's at
Mine was doing the same thing. harsh shift from 1st-2nd and shifting into overdrive sometimes. I just took it to the dealership and they said it was the 1,2 accumulator and that also should fix the overdrive problem too. i should know in the next day or so.