Sounds to me like the climate control module, been there done that, bought the shirt. Similar symptoms to mine a few years ago. Cost about $190. Try these guys, they not only rebuild and sell these units but will help you determine if that is really your problem.
http://www.gdl-online.com/begin1.html
Good luck
__________________
Doug
'83 300SD "Old Gold" 170,000 mi.
'82 380SL "Green Machine" 190,000 mi.
Some days you're the pigeon, some days you're the statue... I'd rather be the pigeon!