Idle issue.

Hi Dr djet, it's Nigel back again.I have had my ECU rebuilt by Bosch Classic,and a engine rebuild. The car starts and runs well apart from when coming off the throttle the revs dive down to almost zero and back up again, The TPS  was wrongly set when I collected and I could not set it to less than 5.6.After resetting the TPS to 2 percent I drove 30 miles and retested it had gone down to 1 percent .Pre last run I set it at 2.8  ,it seems u nless the idle is at just over 1000 rpm the idle wants to keep going too low.After another motorway trip I came home left the car for 10 minutes  did the usual hot start routine  ,and whilst opening my garage to put the car away,I could hear the engine surging   ie reving up and down quite slowly .What do you think is the cause ? Thanks Nigel