I had the same symptoms shortly after I got my 2001 3.0L except for the idle was ok. The truck had 86k on it and appeared to be poorly poor maintained, just driven. It was ok until I did some rapid oil changes of motorcraft 5-20 synthetic blend every few hundred miles until it cleaned up. After the fluctuation happened (only the one time) I changed to Castrol Edge 10-30. The engine used a little oil at first 1/2qt in 1k at first but once I changed to Edge it went 10k without any usage at all. I changed the filter at 5k and at 10k changed oil and filter. I left the 10-30w in over winter without any problems. Just let it warm up good before going anywhere=no problems for me.