I have a friend with a subaru outback 2.5 and it starts and runs but you start to go down the road and step on the gas and it wants to cut out and die.

let off the gas and it starts to comeback and run ok.

It has new plugs and wires and we tryed a coil pack and that didnt help.

the exhast is ok not pluged and no codes.

Checked the fuel filter and thats ok.

TPS?

my 95 2.2 did the same thing , coolant temp sensor, real pain to get to ,

To verify the coolant sensor is the trouble you could watch the voltage change while you monitor it with a voltmeter at the connection to the ECU. This very well could be the trouble. Other things to check are the TPS and MAF sensors.