Overheating 3F(FJ80 landcruiser) when idle

My engine seems to overheat only when idling for long periods of time (shorter periods if air-con is on). The engine has been overheating like this since i bought it a few weeks ago, since then i have replaced the radiator, fan and fan clutch, thermostat, checked for vacuum leaks, etc. It had a new water pump when i bought it. These have made the engine run a bit cooler but nothing has STOPPED the overheating.

I was thinking that it could be ignition timing (however checked this and adjusted it to no avail), I then considered the problem might be that the engine has overheated before and there could be problems with the head, however the engine (i believe) is running to smoothly for this to be the case.

The engine has had a gas conversion, and it does overheat (the same) on both.

I dont seem to have a HAI (Hot Air Inlet) valve on my air intake so i doubt that it is recycling exaust air and this causing the overheating,

however the normal (cold-air) intake is sourced from the inside of the front fender. This whole area gets extremely hot when the engine is running especially when it gets close to temp on the temp gauge the red area, so i was thinking that maybe it is just getting to hotter air, and this causes a loop

Hot air -> hotter engine -> hotter air -> hotter engine -> hotter air again, etc.

Is this normally where the air intake is ?

NOTE: If the bonnet is up the engine does not overheat... only when the bonnet is closed.

I am using a 15W-40 grade oil, many books recommend 10W-30 so could the 40 (ie hotter running oil) be making the engine overheat like this ? (I am just about due for a oil change so i'll post that back here whether it makes a difference or not)

N.B. - the engine does backfire OCCASIONALLY(really only on petrol), if this helps in anyones diagnosis.

Thanks any sugesstions welcome,

Kallum