Truck stalling in Hot Weather, any suggestions??

[JoetheMobster]

Well, my truck seems to be very unhappy with the 30+ deg C weather we're getting here, and my truck stalls after a stint of driving. When the day cools down, I have no problems, but in the heat it stalls. I've let the truck idle for periods of time, and never had it stall on me, but if that sun's shining, the truck keeps stalling on me after about 1/2 hour of driving.

After a stall, I have no problems starting it up again, and if I drive it in 1st, or second while keeping on the gas, it seems to be fine. When I hit third though, and the rev's are low, that's when the stall seems to come.

When I switch the fuel over to CNG, it's like riding a MAD rodeo bull, in all gears

Any suggestions? The air filter might need a changing, but it's not too horrible. I should change the spark plugs, they're in need! Thing is, would this fix anything? Remember, the truck runs great in cooler weather (late afternoon, 5ish) but at midday, it's growly!