I'm no expert on auto's Max but after the trials and tribulations I've been through with my 4HP20 (which is the slightly older bigger brother of the AL4) I would say that your abrupt downchanges which feel like it is slipping into "neutral" are definitely the gearbox and they may not be a good sign...it could be early signs of impending failure
I'd agree with Jim that the box is struggling to regulate the pressure accurately which will lead to rough changes as the clutch engagement overlap can't be controlled precisely. This can lead to it feeling like its in neutral part way through the change with it suddenly grabbing as the clutches engage late.
It could be wear or it could be really dirty oil or a bit of both. Auto's are extremely sensitive to the state of the oil in the box and both the AL4 and 4HP20 are "sealed for life" using the same unique Mobil/Esso LT71141, which means by the time they get to 100,000 miles or so with never having had the oil changed the oil is disgusting, totally black and contaminated. (essentially "sealed for life" is a bit of a con, "sealed until warranty runs out" is closer to the truth...
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First thing to do on an auto box that is showing erratic changing behaviour is an oil change. Because you only get 1/3 to 1/2 of the oil out each time you drain an auto gearbox you might have to do more than one change of oil if the oil is really bad, normally you would leave at least 100 miles driving between each change if you're doing multiple rapid changes to allow time for the new oil to homogenise with the old. You might not notice an improvement in symptoms until the 2nd or 3rd change, so if the oil is really dirty and you don't notice much difference after one change, don't give up.
I had problems with rough shifting and a very worrying "groaning" or grinding noise under load at low RPM that was getting to the point that I thought the box was not long for this world, but 4 oil changes over a month or so have pulled it back from the brink and it has been working fairly normally since with nice smooth changes.
Have a look at page 21 of my massive thread below for pictures of the oil that came out after each change and you can see just how incredibly black and dirty the oil was (probably burnt in fact) after only 100,000 miles:
http://www.frenchcarforum.co.uk/forum/v ... &start=300" onclick="window.open(this.href);return false;
One oil change showed no improvement, 2 changes showed a small improvement in symptoms, after the 3rd and 4th change of oil the symptoms were all gone and have remained in remission for for the three months since then.
If you're changing the oil on an AL4 there is a special procedure to be followed (which Jim has a copy of, if not I should be able to dig it up) and you'll need access to a Lexia to reset the oil wear counter after each change. (You don't actually reset it to zero, you subtract a certain value off for each oil change so the gearbox can keep track of how dirty the oil is) You must also use Mobil/Esso LT71141 and nothing else.
I would also recommend doing an "auto adaptive reset" after you're finished with the oil changes which is also done with the Lexia. This resets the ECU's parameters controlling how the clutches are controlled, forcing it to "re-learn" the characteristics of the gearbox, which will temporarily result in harsh shifts while it re-learns (maybe for 20 miles or so) but after a while it should result in smoother gear changes.
Changing the oil really is the only "user serviceable" thing that you can do to fix an automatic gearbox so its worth trying, its also best done as a preventative measure at the first hint of any symptoms, or even without symptoms if the car is new to you, has done a good mileage, and the oil condition is unknown.
If you leave it until the symptoms get really bad it's probably too late as automatic gearbox failures tend to occur through a domino effect where one thing leads to another in a vicious circle until it drops dead. For example the oil might be dirty with metallic particles floating through it from wear, these particles may lodge in a control valve that prevents the main rail pressure being regulated properly or may clog the filter which also causes a loss of pressure, the loss of pressure then leads to slipping clutches which overheat under load and shed lining material into the oil, this lining material then further clogs the filter or interferes with the control valves etc causing the situation to rapidly spiral out of control until something breaks. Catch it early and it might be perfectly alright after a few oil changes.
