I have a 99 ford ranger with a V6 4.0 engine. recently i have noticed that, when the engine is idling, every 10 to 15 seconds it will rev up to slightly above 1000 rpm and then go back down to normal. The engine will do this if it is in drive, neutral, reverse, or park and it seems like i have been going through gas much faster. does anyone know what is wrong or if anything is wrong at all?