fix or trade? HELP subaru problems.....

we purchased a 200 subaru outback limited legacy... with 100,000 miles on it... a month
after driving it, it started to back fire and stall.  we had the spark plugs, spark wire set,
fuel filter, air filter, o2 sensor, catalytic converter/ gasket... oil changed/ fresh anitfreeze,
thermostat and seal replaced. i drove it for 2 weeks, and it is almost stalling when i slow
down, i put it in neutral and rev the engine and put it back in drive when i have to move....
i have had it in the garage all weekend .... not sure if i need a new timing belt or if its time
to get rid of it... please advise... thanks!! we were hoping to use it for another 150,000
miles!!