I have a 99 Ford Explorer XLT AWD with 145,000 miles, well taken care of by previous owner. The problems I have noticed are (1) when idling, you will hear a "tick" noise and the RPMs go up like 200-300 RPMs briefly and then back to where it was before. It does this rhythmatically. I had a visual inspection done at a dealership after purchasing it and they said it was nothing. ??? and (2) when accelerating, i feel hesitation @ 30mph as if it lags/drags a bit before speeding up. I thought maybe spark plugs/wires but was told probably not. I had the transmission looked at and they couldn't find anything wrong. I made them stop looking when they offered to go in and try filing parts/pieces if needed and seeing if that fixed the issue. i said no because if they don't know what's wrong and they are a transmission repair shop, then I prefer they left it alone. I also should mention I have a delivery route I do 7 days a week with constant stopping and going @ 10 - 60mph.