My 740 sedan stalls periodically in "Drive" and in "Park." It will either just stall out or it will rev up wildly several times and then stall. Sometimes it wil rev wildly and then be fine. It also seems to idle when in drive, at different speeds. When it is working fine it stays at about 900-1000 rpm, but when it gets crabby it goes down to 500-600. The first time I noticed the problem it was differnt. It started with the engine starting to stall then it would catch itself and rev up. It would do this several times and then either stall or just run fine. I had the exhaust manifold repaired and now it does what I described above. Could it be another problem with the exhaust system? Someone recomended a new idle motor? Does it just need a tune up? Any suggestions?