Every time I start up my '93 4.0l ranger, it'll start, go to high idle, then the rpm will drop until it almost stalls, after it almost dies the rpm will jump back up to high idle like it normally should. I thought I had a drain back issue with my fuel pump but it'll do this several times in a row when it's around 50-60 degrees in the morning. No check engine light, fuel filter and air filter have been changed.