thermal expansion valve on a heat pump

I have been having trouble with our Armstrong heat pump. It will run fine for awhile and then i will come home from work and it will be 80° (or warmer) in the house (thermostat is set @ 73°). After several service calls from 2 different repair companies (we have added a hard start kit, replaced the compressor, and run capacitor) the latest opinion is that we need to change the thermal expansion valve. Fortunately the unit is still under the original 5 year warranty (that ends in a week) but this repair is going to cost $300-$400 in labor. To keep the system going (or going again when it has stopped) I have to turn off the unit (either at the thermostat or at the breaker) wait 5 to 10 minutes, then "restart" the system. Usually when the system restarts the condenser fan and compressor start at the same time. Sometimes I have to repeat this a few times to get it going. Then it might run for 2 hours or a week before the compressor drops out (It can drop out during the cycle or when it tries to run again the compressor doesn't kick in. What's your opinion?