1998 Ford explorer low idle

My 1998 Ford explorer has about 140,000 miles and it idles too low, around 400, and it does it worse when it's cold, or the weather is cold. It has a manual transmission, which makes it more diffcult to keep from dying. I tried adj. the idler screw, but that really didn't help. It runs fine, just idles to low. I replaced the spark plugs a year ago. The engine is a 4.0L I think. This has been this way since last winter and doesn't seem to have gotten any worse, but with winter coming on I know it will be more hassles.
Thanks,
Joe