Hi. I have the same car.'87 300D. LOVE IT!There are a few things you should look into. 1st I would look at the A/C compressor.If it was in need of service there's no telling the internal condition.It might still work but if it is starting to sieze it could get ugly.
2nd,when I first bought my car it was sluggish all the time.Off the line,passing,accelerating up a hill,ect. I was sort of stumped but having diesel knowledge I knew one thing for sure,lack of AIR or FUEL or BOTH will kill a diesel.Soooo I spent $60. and changed all the filters,Air,& both primary,secondary F.F.The difference was night and day.I can lay rubber from a stand still and chirp the tires going into 2nd gear.She pulls to almost 70mph before hitting 4th.(if I desire).I'm getting 30mpg around town and 35-38mpg hwy.In summer of course.
Finally I bought a can of lubromoly diesel purge which you run straight from the can to the injection pump and that smoothed out the rough ide when she was cold.I bought her from the original owner with 111K.She now has 130k.just starting to get broken in.I figure with close maintenence My 12 year old daughter will be getting her when she's 16.
I would also change the tran fluid/filter and go with the Redline synthetic.It may cost a few $$ but I have heard good things about it helping trans shifting problems.Good Luck. JOE