Engine vibrates when relase accelarator

[chhigo]

Dear Friend

I have a BMW E30,316i 1989 M40 engine car. When release the accelarator, engine RPM going to 650-700 and badly vibrates the car. After vibrates, engine going back to normal RPM(800-850) and runs smoothly. This is happening alway when release the accelerator padel. This problem bit verse when A/Condition is on. Please give me some tips/solution to resolve this matter.

Thanks
Best Regards
Channa