Dynaflow problem(in Sweden) - please help!

Dear Buick friends,I have a problem with the Dynaflow transmission in my 1955 Buick Century. Around where I live in Northern Sweden nobody seems to have worked with such a car or trans so itīs no game to own and preserve a such car here. Well,itīs like this: when starting the car after it has rested for a while,(more than 10-15 minutes)no matter hot or cold it seems,the clutch doesnīt engage when shifting from P or N to D,yes it will if I rev up the engine but then it will engage rather brutally because of the higher RPM. If I start in L or R, function is totally normal,that always works and after that the shifting to D is also normal and will be as long as engine is running. Then the same problem occurs again after having the engine stopped for some time. Shift linkage is adjusted acc. to shop manual,trans. fluid used is Mobil DexronII,all functions are OK once the D is working,no noises,no slipping or other problems. Whatīs wrong? Please give me a hint if you can....... Thanks!