urgent help neede for newbie!!!!!!!

hi all, im new to the bmw world and have no mechanical knowledge whatsoever and im trying to get some information on my 92 325ise, i recently purchased and since the purchase have been alarmed by a
seriously loud thump coming from the back whilst acceleration/deceleration and gear changes, sometimes, when the throttle is released the thumping can actually rock the car slightly, similar to kangaroo hopping but not quite as violent. i spoke to the dealer about the problem and he assured me it was a common problem with e36s and is the rear bushes??? as the car had passed an mot on the day of purchase i didnt worry to much about it, however the problem seems to be increasing. does anyone have any ideas as to what the problem may be or has suffered with similar problems? also the idle keeps fluctuating between 500-1500rpms? all help would be gratefully recieved, thanx
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