Nissan 4x4 problems

I have an 86 Nissan 2.4L 4x4 king cab 5-speed with 145,xxx miles on it and I am having problems with the motor I think. In the mornings it is really cold, around 40 degrees and I start up my truck, let it warm up for about 3 minutes and then hit the gas to go and nothing happens. It is running at around 1000 rpm's and over a period of time, about 5 minutes, idles lower and lower until it dies. keep in mind that while it is getting lower an lower I pump the throttle and nothing happens, it doesnt go up even 100 rpms. When the whether is warmer though it has no problems at all. I have already tested the fuel pump,replaced the fuel filter, the air filter is fine and I have ran premium gas in it since i got it about 6 months ago. If you have experienced this or have any ideas, they are much appreciated.

-Nathan