I'm bummed, my baby is hurting and I don't how to make her feel better... I recently had the thermostat and fan belt changed, then in less than 200 miles, the engine started acting weird. On a particulalry long, steep hill in northern NH, the check engine light came on and she started to run rough. I figured the cold might be part of the problem (-14F), and I assured myself that when I get back home (in MA) that I would have it checked. Well, I changed the plugs and wires, and replaced the rear O2 sensor. The engine still ran rough when accelerating, and dropped in RPMs when Idling to around 500-600 to almost stalling. Also, there was a knocking, not a tapping like water in the fuel, but knocking in the engine at all times (that it was running). I had lost alot of power to climb hills and accelerate, too. I dropped it off at the Ford dealer, the diags said that cyclinder 4 & 5 were misfiring and they suspect the timing chain has problems. They need to remove the front of the engine to see what the problems are ($400). Then if the problem is timing chain only, then another $8-900 to fix that, but possibly, on the high-end, $3500 for a new engine. I'm in shock. Something doesn't sound right to me, and I need more opinions from those who know.