What do you mean by it drops out of gear?  Do you mean that if you take you
foot off the brake at an idle, it will not move forward unless you give it
some gas?  If so, clutch wear.  How many miles on the tranny?  Does it shift
through all of the gears properly?  Do the lockup work?

 "I have A 92 buick leSabre that drops out of gear when ever I stop. I
have replaced fluid and filter and added lucas tranny additive and that
stopped the problem for about 6 months but it has recently begun again.
Have thought about a modulator but dont really think that would help.
anyone have any other ideas or am I about to drive it to the scrap yard
?"
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 I bought a '92 Century as a winter beater  with automatic overdrive
which suddenly began to do exactly what yours is doing.  It performs
flawlessly around town and down the highway but when I come to a
stoplight it will act like it's not in gear when I hit the gas to take
off.
When it first started, I would just bump it up into neutral, and then
right back down into drive (overdrive in this case) and it immediatly
'resets' it ( for want of a better term) -- and it then works perfecly
normal until next time I stop.  I've kind of gotten used to it, like
pushing in the clutch and hitting the shifter at a stop with a stick.
 At any rate I would guess doing _that_ is much easier on the trans
than revving the engine until it slams into gear.

 I posted in the transmission forum and one of the techs there said
it's likely "an input clutch problem" and that, "yes, you can drive it
like that until it stops moving."  And that's what I've been doing for
about 3 months now.
Since it's been a particularly harsh winter here with tons of road
salt, I drive it everywere i go, several times 100 miles from home.
 The thread is still there in the transmission forum at batauto.com,
the heading is something like "92 Century goes into neutral at stop
signs."

CC&P from a thread in the transmission forum I'd posted in for a similar
problem, similar trans.
 I had stated that when my trans would neutral out at stop signs, I
would put it in manual low to get going again.  Since then, I've taken
to putting it in neutral and then back to overdrive to get going again,
instead of manual low.
 
 "You may have had parts go bad but now you most likely have and input
clutch problem, you use 3rd clutch & third roller in man low which will
over ride the input clutch. I do not recommend Trans-x for any reason.
The vacuum modulator should be checked but it is probably too little too
late, disconnect the vacuum line from the modulator and look for any
fluid inside the vacuum port. there is nothing electric that could cause
a "no forward" in that year trans. I have never heard of adding extra
fluid to a Gm unit except to check for "filter drop", where it is common
practice to add an extra quart to Ford because the factory under-filled
them on purpose. The "TV cable" on this trans controls shift timing
only, the modulator controls shift strength but even if the cable were
broken completely it would not neutral out at a stop. I just don't see
anything "Minor" that could be causing your issue. Sorry, Transman"