Problem is the tach works great with air con off. When on, at idle, tach registers zero. Upon reving up in gear, starts reading again at about 2000 rpm and continues until 3500 in hi gear when it again begins to jump all over. Tach revs normally from 2000 to shift point thru the gears, just bounces when it hits 3500-3600 in hi gear. Any ideas

Regards

Run-em