Odd idle oscillation warm start

[imapaintballa]

Hey everyone, I'm new to these forums, but this is a very strange problem that I'd like to fix. Whenever I cold start my 1996 4.0 XLT, the engine idles around 1200 rpm, then lowers to around 700-800 rpm, which is good. Say I had to go into the store and as soon as I come back out to my truck, I start it up [with the engine warm] and it seems to oscillate below 500 rpm from 400-500 rpm back and forth. This happens for a good minute before going back up to 700-800 rpm. I'm worried this could be a fuel line problem.

For the record, the engine has 99,678 miles and the air filter is not dirty. Although, it'll need replacing in another 2k or so.

Thanks for looking! -Adam