300D turbo slowing down!!

love my 300D turbo, 82' 225k miles. lately, it's been slowing down on the highway, can't get past 65mph, when i first get on the highway, it feels a lot stronger, pulling to 75-80, but starts slowing after prolonged driving. could the presure in the turbo be dropping? other than the basics, i am not very knowledgeable about the mechanics of engines (diesels especially), so any help would be appreciated.
second,can anyone please inform me of the routine maintenance needed for diesels? usually just changed the oil, so far nothing has broken down, but i feel i haven't kept up with things.
lastly, my trans slips trying to get into 4th when first driving after a long rest. can a trans service take care of this or is it time to rebuild??? thanks a bunch for any and all suggestions.