I have a DeWalt DW621 router that is about 6 years old. Not as variable
speed model. I have become increasingly unhappy with it. I never did like
having the plunge lock work by twisting the handle. Very easy to tip the
router while adjusting the depth of cut. It also does not easily lend itself
to gradually ( say 1/8" at a time) increasing depth. Now the plunge
mechanism has started to slip on me, so after making 2 or 3 1/2" deep cuts,
the next one may go to 5/8" or even more. Bummer.

Anyway, is there a modification that can be performed or some way to better
control this beast?

Well if you find the base unstabile - get a round base for it
- say from Pat Warner.

For progressive depth cuts, there is a depth stop turret with
three stops you can set on the 621 (and other plunge routers)
for progressive depth of cut cuts.

As for the depth stop rod slipping - the 621's plunge is very
smooth and the return spring isn't overly strong - so you don't
have to really slam down on it to get it to move. I don't understand
why the rod should be slipping and allow for deeper than desired
cuts unless you really slam it against the turret stop - or really
really really lean on the router while routing. Did you by any chance
put oil, grease or WD40 on the shaft?

As for the Twist To Lock handle arrangement, I like the fact
that I can retain control of the router with BOTH hands and
NOT have to reach for a locking lever.

charlie b