Defender 90 backfiring &&&&&&
 

My wifes Defender 90 is having a couple of problems. Can anyone help me figure this out. If you rev the motor an 3/4 throttle or more it backfires constantly. Also after driving it for a while if you turn it off it wont restart for a couple of hours.
This is a 1997 defender 90 (4.0) with 62000 miles. We use regular unleaded gas.

I have already changed the
Plugs
Plug wires
Inline fuel filter
Added a fuel additive to clean injectors