Slipping transmission.. what can I do?

I have a 1995 Ford E250 3/4 ton cargo van with 176k miles. 3-speed C6 auto
transmission, 4.9L L6 engine. The transmission has been slipping every
once in awhile when driving uphill between 20-25 mph. By slipping I mean
there is a loss in velocity & acceleration and the engine seems to 'race'..
i then let off the gas pedal and just barely accelerate until I make it
over that 25 mph mark and it seems fine afterwards. I just changed the
fluid, filter, and cleaned the pan out and am still having the problem. The
problem seems to get worse once I've been driving for awhile and heat
builds up. Are there any sub-components of the drivetrain/transmission
assembly I can check out to hopefully 'bandaid' or even possibly fix the
problem before it gets worse? Does this problem always lead to complete
transmission failure? The driveshaft was just balanced/straightened out
and all the splines on the yoke/output shaft are solid. Would appreciate
any help here!.. I have haynes & chilton's manuals but please keep in mind
im a novice mechanic.

thanks
Jay

check trans fluid level on flat ground with engine running warmed
up.high mileage usually euals worn oil pump.lower trans
pressures.adjusting kickdown for hiher shift points will increase
pressure.but if you already in high when this happens and it aint due
to an attemped downshift from hill grade then part drasin and refillup
with 30wt hyd oil may be a fix.this will boost pressure.if it aint doin
it cold then then as oil warms and thins out it is leaking internally
more you could also try an oil stabilizer which improves viscosity
characteristics.

Did you do a band adjustment when you changed the fluid and filter?
(The old C6's, at least up to 1994, had an intermediate band adjustment,
and just assumed the newer ones are of the same general design.)

Your transmission has enough mileage on it that, I suspect, the
clutches are worn down. If so, an overhaul is probably in its future.
No amount of additive, etc, will help if your clutch packs are worn out.