After doing head gaskets which included a valve job am having difficulty when starting out from an idle. This is an automatic transmission. Car starts right up and idles without missing. No changes when put into drive. However, when you push the accelerator down it feels like spinning the tires, once rpm's reach approximately 2000 the engine and car accelerate properly. This occurs at a stop, and when going up a steep hill. After the rpm's reach 2000 everything seems to be working properly. Going down the highway is not a problem with normal acceleration. I have replaced the throttle position sensor her, idle air control sensor, front and rear oxygen sensor, and cam sensor. I have replaced the spark plugs, and the spark plug wires with genuine Subaru wires. Occasionally everything works normally. The car has been into 2 different shops, a lot of money, and the problem remains. The second shop replaced the battery cable end and the car ran perfectly for 2 days. This is driving me crazy. Any suggestions? There are no engine code.