Chevrolet Repair: Acceleration issue, chevy impala, impala ls

Question
Hello,
I have a 2004 Chevy impala LS 3.8 with about 150K miles. If I am at a stop light or at a dead stop and then I accelerate to take off the car will jerk. I am not sure whether to describe it as a jerk, a hesitation, or a slip. When it happens I always immediately take my foot off the peddle and then I reapply my foot to the peddle to give it more gas. When I do that, the car accelerates fine. This has been happening on and off since January of 2010. It is happening more frequently now and the other day the "jerk" was quite intense. It only happens after I have been driving for a while and the car has had a chance to warm up. I have not paid attention to the RPM’s when this occurs because it always happens when I need to be watching where I am going. LOL

Some other issues I am having with the car (which may or may not be related to this or causing this) are as follows:
1)   I get an engine code of P0455. I have been getting this code since June 2009. I replaced the fuel filter in November 2008, the evaporation solenoid in July 2009 and the thermostat and throttle body gasket in May 2013. My air filter is fine. At this time my mechanic mentioned there is a little coolant getting passed the head gaskets.
2)   The engine is running slightly hotter than normal. The temperature gauge after the car is warmed up runs at the highway mark (never goes higher) where as it used to run a hair lower than that. The car’s fuel efficiency has also slightly decreased. I put on 150 miles a day going to work and I monitor my gas mileage so I know it has decreased.
I really appreciate any input!

Maddy

Answer
HI MADDY,I WILL START OUT WITH THE PO455 CODE. THIS WILL HAVE NO BEARING ON THE WAY THE CAR RUNS. THIS IS FOR A LARGE EVAP LEAK IN THE FUEL SYSTEM..VAPOP LEAK NOT RAW GAS..THIS WILL NEED TO BE TAKING CARE OF IF YOUR STATE DOES EMISSION TESTING..WILL NEED TO PUT SMOKE IN THE EVAP SYSTEM TO FIND THE LEAK..IF YOUR ENGINE IS STARTING TO HAVE HEAD GAKET LEAKS I WOULD ADDRESS THIS SOONER RATHER THEN LATE. THIS WILL MAKE THE ENGINE RUN HOTTER THEN NORMAL AS WELL.THIS WILL ONLY GET WORSE..AS FOR THE RUNNING ISSUE THIS COULD BE FROM A FEW THINGS, MASS AIR FLOW SENSOR,TPS SENSOR[THROTTLE POSTIONING SENSOR]AND LAST IS A FAULTY THROTTLE BODY UNIT..THIS WILL NEED TO BE TAKEN TO THE CHEVY DEALER OR A SHOP THAT HAS GM SOFTWARE TO RUN A DATA SCAN [NOT JUST CODES] ON THE PCM [POWER CONTROL MODULE] TO DIAGNOSE THIS PROBLEM CORRECTLY..WITHOUT THIS I AND YOU WILL ONLY BE GUESSING.I WISH I COULD BE OF MORE HELP BUT THIS IS SOMETHING I WOULD REALLY NEED TO DIAGNOSE IN PERSON.....GOOD LUCK TO YOU,,,TOM.....