I have a 1989 Ford F150 Lariat with a 302 V8. when i let the truck sit for a few weeks at a time it has a weird idle upon initial startup. It will idle between 500-1000 RPMS and has no constant to where it stays. the tach needle goes up and down in between these two incriments. plus when ever i shift the truck from park to drive it stalls out (By the way this an automatic transmission in my truck). so i restart the truck and bring the RPMS up to 6500 and hold it there just for a split second and it fixes the problem until i let it set for a few weeks and it has this problem again. The motor is original and has 180,000 miles on it and has developed a lower end rod knock and i am planning on rebuilding the motor in due time. please let me know how to remedy this problem so i can fix my baby

 

It is 4-Wheel Drive if that makes a difference.
And it is Fuel Injected also if that makes a difference

 

Have you tried anything along the lines of a tune up (i.e. plugs, wires, fuel filter, air filter, etc.)?

Also, it may not hurt to run some Seafoam in the tank and engine. It could help clen out partially blocked fuel injectors and may help the knock a bit.