M119 Engine Loss of Power when Drive Gear engaged and AC

Hi I would greatly appreciate any assistance, my 1992 R129.066 with
M119 engine - I recently did an engine Head overhaul which was successful, however after about 500 miles I noticed a slight miss and rough idle as you gently pull off, (I had also replaced new spark plugs about 4000 miles ago), I replaced the Spark Plug wires using Bosch OEM wires the ones with the red ceramic type connectors 2 weeks ago. On driving the car for about 100 miles i noticed there was a missed that became more pronounced which was more severe more so at lower rpms its most noticeable (it sounds as though there are about 1 or 2 cylinders misfiring) I had a detailed inspection and all hoses and lines are correctly in place, and as i said when the car is started cold in the morning there is virtually no power if i engage the AC with the gear in Drive the rpm drops right down as if it wants to shut down. (note the idle air mixture is fine). Does anyone have any suggestions as to the cause based on similar experience, could it be the lower range fuel pump breaking down or perhaps the Idle Control Valve ?? comments welcomed thanks.