M15 tranny issues

I have a 2000 Impala, LA1 motor, M15 RDB tranny with approx 84K miles. When I accelerate with 2/3 throttle or more from a stop, it feels like the tranny slips. This also occurs at speeds up to about 15 MPH. I changed the filter and fluid twice, using GM filter and Castrol Dexron 3 fluid on the 2nd one. The pan had a reasonable amount of very fine grey residue and the original fluid smelled appropriate for its age (not burned). There were no large particles in it either time. Gearshift position doesn't change anything. Otherwise, tranny shifts are excellent and can't get it to slip at full throttle upshifts or downshifts. Also, engine speed stays appropriate for the speed of the car when this is happening...it doesn't seem to over-rev, the car just shudders as if the tires had marginal traction (were slipping a little). Doesn't seem to matter whether I'm going staight or turning in either direction. Any ideas?
While we're on the subject: What does the identification code (RDB) do for the tranny? Can I only use that tranny if I need to replace? Or, with a slightmod/change, can I use another code/year?

Man, I have the exact same issue. Doesn't always happen, but it's alot more common after I turn off the AC. Mind you, my car is possessed by demons. For example, my radio, once or twice, kept turning off after I turn it on. My signal switch sometimes refuses to signal. I had it on "signal left" and nothing was happening. I kept it in the same place, turned of the car, turned it back on, and BOOM it works again. So I can't clearly say it's transmission slip. my transmission has some problems, but it doesn't make sense that it only slips once in a while. It's too bad these cars don't have a tachometer.

Hello, anyone else out there? How about a Moderator? Can you move post to "non-specific"?, or any other cars with same tranny. Someone has to know what's going on.
INF3RN0666, it happens more w/o AC because engine has more power (and torque). Electrical problems like yours tend to be a simple loose connection, trace it out from your radio. If the radio retains station, volume, last setting memory, etc, check out the ignition-switched power wire from the back. I can look in my Helms manual if you don't have the resources to find which wire it is. Let me know.

Go get it scanned look for a DTC P1811 or long shift adapt times. Its probably a bad PCS

I don't have any trouble codes on my car, but what's a PCS?

As for the radio problem, it hasn't done it in months. Mind you, I had that windshield leak and I would get water on my vinyl flooring. So some wires might have been corroded. Would the wire be under the passenger seat?

Thanks Troy1, was concerned that it was a pressure issue, but shifts are excellent. Got to get the code(s) read.
INF3R...the wire I mentioned is in the dash, but if you have a factory power amp between the rear speakers, the dash unit needs the interconnecting wiring between them to be good. And, yes, this wiring runs on the passenger-side floors. I believe the white Amp sense wire needs to be (not grounded) to allow full dash-unit functionality.