May be obvious question, but have you cleaned all the connectors?
Just because it works on another Amiga you may have dirty connectors.
Also what revision is the A3640? as A4000T needs revision 3.2 although it will work with older revision. This cards are prone to overheating as well.
The A4000T (Tower) also has a crap fan, i would recommend replacing this with a more powerful fan.