My car has almost the exact same problems...runs fine when cold, but once warmed up, it behaves the way you described. I took it into the shop, so I should have an explanation for this by monday. Oh yeah, the car is a '90 300se. At the moment, it sucks. I posted about this a few days ago, and I was advised that it could be a major vacume leak... I think the thread was called "300se hesitation."
I just spent 2 hours scraping my knuckles fixing the rear door problems and this is how the car repaid me. I think it wants to be sold...
I'll write back as soon as I have some information.