2000 GMC Yukon engine looses power and sometimes surges

Please, help we've spent over 1400 thus far. Fewer problems when it's cool outside. More when it's been running a while or in the heat of the day. The engine will sound like it's missing out for about 5 minutes and then it will flat out fall on it's face. It doesn't die, but it looses all power and the more gas you give it the weaker it seems. When you pull off the road and turn the key off it will usually reset. In other words, there is power when you start it back up for a while. This could be for 5 miles or 25 miles. It does seem to be happening more often though. GM charged us $70 to look at it but didn't give us any answers. Replaced: fuel pump, filter, air filter, plugs & wires, distributer cap, rotor, coil, MAP and mass air flow sensors, throttle pos. Control, cat. Converters. Getting ready to replace ECM with little hope. Has anyone out there had this problem?