Wierd Rough Idle

I searched "rough idle" and "stalling" but nothing matched the following conditions???

While following my wife in her 1989XJ (4.0 Auto), it ran fine for 15 miles. Then it gave a little white exhaust smoke and began idleing very rough, surging low to high idle then stalling. very minor tick in the top end while idleing rough...clears up when idle surges up??? Any ideas? Super tech secrets?

Get a multi-meter and check out this site, diagnostics for Renix
http://www.lunghd.com/Tech_Articles/Engine/Basic_Sensors_Diagnostics.htm
Each sensor is really easy to check with a meter. Good Luck.