Impala - Chevrolet :: Transmission Slipping When Shifting From Reverse To Drive

2007 Chevy Impala3.5L 6-cyl Flex Fuel engine w/ 4 speed auto trans52,800 miles

Extreme driving conditions maintenance schedule kept.Transmission standard service done @ 32,800 miles which was 20k ago.

PROBLEM:

1) Twice in past three days, when shifting from reverse to drive the transmission has slipped, revving hard with rpms spiked above 3,000 for about 2 seconds before engaging the forward drive gear.

Normally I come to a complete stop of all reverse motion with foot fully on the brake pedal before shifting. Admittedly, I got sloppy the first time noted and still had very slight reverse motion when I shifted.

2) Several times late today, while driving uphill at approx 35 - 45 mph, when the car downshifted there was an extremely brief moment of transmission slippage with spiked rpms. It was so brief each time that at first I thought I'd imagined it until it happened again. Then it did it once on a level surface in a parking lot when having shifted from reverse to drive without problem and was gently accelerating the transmission again slipped for just the briefest moment when the normal automatic shift point was reached. So far, all incidences while driving have been so extremely brief that it was the merest hint of a hiccup.

3) When bringing the car to a complete stop, such as at a stoplight or when parking, as the transmission automatically shifts for the braking of speed, sometimes there is a minor but noticeable thunk sound seeming to come from underneath the car, I'd judge to be somewhere about under the level of the dashboard or slightly forward of that. Could that be from a worn or broken transmission mount? And, if so, could that be somehow causing the slippage problem?

4) This transmission has always, even since new, allowed more roll after being shifted into park, the engine shut off and my foot taken off the brake than any other car I have ever driven. If I forget to engage the parking brake quite tightly, the car will roll forward quite a few inches before the parking pawl holds it, even on the level. I've been told both by the dealership shop and the independent mechanic shop that there is no problem and that this particular engine/transmission combination tends to do this although my particular car seems to do so somewhat worse than average.

ACTIONS TAKEN SO FAR:

1) Have made an appointment to take the car into the mechanic.
2) Will check the transmission fluid level per instructions in the owner's manual tomorrow when I have daylight to see what the fluid level and color on the stick looks like.

NOTE:

1) I periodically check the transmission fluid level and it has never been low or discolored. The shop checked it for me, along with making sure everything else was road trip worthy, for an emergency 1,000 round trip in late November, just three months ago. No problems then.
2) No evidence on the garage floor of any leakage. Garage floor is dry.

QUESTIONS:

1) I presume the first step is for the mechanic to check the fluid level and examine for any leaks?
2) If the mechanic cannot duplicate the problem in a test drive, then what?
3) If the problem cannot be duplicated and no apparent cause found, I presume that the prudent course of action is to go ahead and have standard transmission maintenance done (i.e. drop the pan, check for debris, drain and replace with fresh fluid) then see if that halts the slippages?
4) Given that this car has already had failures of the power steering pump, water pump, and part of the wiring harness, and has a fuel pump starting to get noisy, despite aggressively maintaining all standard/preventive maintenance, that perhaps it is time to think of replacing this car, especially if there proves to be any truly major transmission problem found?