87 626 engine missing at idle

i have a 87 mazda626 2 liter non turbo. my prob is a terrible miss at low speed or idle. during acceleration the engine works great however a compression test revealed the two center cylinders both down on compression.other than a burnt valve does anyone have an idea as to what might have caused this to occur almost overnite. i shut it off working great ..went out an hour later and it was like a diferent engine. plus it guzzles gas now. thanx for any help anyone may be able to provide