I have a '90 240DL with 255,000+ that shifts hard from 1st to 2nd gear (enough to give you whiplash!) all the time but occasionally slips completely out of gear. I have to slow down and it will go back into gear. Is this something simple to repair or will I be replacing the transmission? Thanks for any suggestions!

Uh, ok how 'bout telling us whether it's a 5-spd or an automatic?
If it's an automatic, banging from 1st to 2nd is a definite sign of age-related wear. They're tough but not immortal. Make certain the downshift cable is correctly adjusted, and not frozen, and then do a fluid change. Follow the "ATF FLush" instructions in the 700FAQ. Easier than you think. If this doesn't help, try a synthetic fluid. If THAT doesn't do it, start saving up for a replacement. You don't need to pay to have an automatic rebuilt- that's $900 vs. $150-400 for a good used working one.
It also sounds like the trans may be low on fluid. If it shows correctly on the dipstick (check when running), try over filling it just a little and see if that helps. Does with some cars. Not a lot- no more than half a quart.
Again, not a healthy sign, you could be in for a replacement before too long.

If this car's a 5-spd (that would be the M47) then *maybe* you've got bad engine mounts, or a loose shifter causing a misalignment of the trans-to-the-shifter-to-the-hole, or it could be losing it's bearings - usually they get "crunchy" before they fail, and it's usually in gears 1 and 2.

Good luck with whichever you have.
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Rob Bareiss, New London CT ::: 86 244DL, 87 244DL, 88 744GLE, 91 244: 808K total

Oh, sorry, it's an automatic.

Well then, you're in luck if it comes time for replacement. AW70's (The 4-spd automatic) are a dime-a-dozen. You've actually got to SEARCH to find a decent 5-spd. And I wouldn't fear a used transmission- there's no reason it won't last as long as your existing one did. My own 86 had a really tough 1-2 shift, which my 87 and 88 never exhibited. I changed the fluid and it seems to have worked through it. It's still firm, I mean you NOTICE the 1-2 shift, but it no longer slams your head into the head rest. The others kind of gently mush into 2nd gear- it's almost invisible. The 87 especially, with 223K or so on the clock has a very "soft" transmission. I don't have good explanations for the differences.
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Rob Bareiss, New London CT ::: 86 244DL, 87 244DL, 88 744GLE, 91 244: 808K total