300cd,

The faster the car goes in first gear before you shift the harder the synchro works, and most likely the more of a hurry you are in which also stresses the synchro. So you may have a weak synchro or are developing one. It also demands more attention by the operator not to over load the synchro. In general, the car will tell you what its natural pace is, and in my experience trying to change that is not worth the trouble.

As noted already, part of the problem can be bad timing and incomplete unloading of the gears due to a clutch that will not disengage fully, which is not an operator fault unless the operator is not depressing the clutch pedal far enough. Another possiblity is the lubricant in the transmission. This is the subject of heated debate regularly, so I am not going to steer you to any particular brand or type. I have identified that endorsing a particular lubricant is the event horizon for the black hole known loosely as lubricant discussions on this forum. Try a search if you have lots of time.

But, my inclination would be to stay at 20 mph or below as the last few mph typically comes at the expense of an assault on your hearing even without the crunch. With a crunch it can be offensive to your other senses as well. Good luck, Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)