When the weather is below 40 out side my 318 runs ruff and chokes out, the RPM go up and down and when you start off it bogs down then reves up and goes up and down. In the afternoon when the outside temp is up it runs fine????????????

 

Check for codes and verify your fuel pressure.

Without further diagnosis I would suspect a coolant temp sensor issue.

 

Since your truck is an ’88, it should have a throttle body on it. I recommend you try the following:

Remove your distributor cap. Underneath there is a plastic plate held in by 2 screws (one on each side of the dizzy). Remove the plate and unplug the wire connected to it. This is you Hall Effect switch. If it gets wet or corroded it can cause issues… so just make sure it is clean and that the plastic plate is not warped. Next, make sure you have no corrosion on the contact in the distributer cap. Lastly, clean and gap your plugs. These 3 things contribute to 99% of the engine problems I have experienced, especially if it is cold or damp out. Its also free to do it, and can be done in half an hour.
Both the air temperature sensor (on the side of the TB) and the coolant temp. sensor can also contribute to this issue. They can be tested, but I usually just replace them and keep as a spare for reliability purposes. I would look into both of these sensors, whether you test them or replace them outright.

Hope this helps.

 

Mine does the same thing, but if it gets below about 10 degrees out it goes away, so I'm thinking its just moisture getting into the distributor cap with the morning frost/dew . . . and when its cold enough that the air doesn't hold any moisture, it goes away.

What happens if you try to drive it right away? Does it drive ok?

 

It still runs ruff and rpm's go up and down and it will keep doing it even after it warms up. Then in the afternoon when the weather is hotter it settles out.