05 suburban p0455 code but think there's more to the problem...

[Kristen]

160000 current miles. recently bought...changed battery spark plugs spark plug wires oil change as well as thermostat. Started noticing idling problems almost like someone stepped on the gas. When in park or while at a light holding the break. Thermostat at the same time was fluctuating and the car was not getting to appropriate temp staying to cool. Changed thermostat. Now gauge works fine but car will sometimes still idle high as well as while driving about 70mph the car feels like it's running out of gas. Light came on for gas cap changed cap light still comes on. Thought to check the erg valve but I'm assuming factory bypass or prior owner bypass. Now the p0455 code is coming up car still is driving good otherwise no other codes...my next check is going to be vacuum leaks???? any input would be appreciated  

[sefiroxx]

Check the evap solenoid. It's supposed to only open at mid rpm to recirc the fuel in the charcoal canister

[riverbanks]

455 is large leak,so cap changed,next is vent valve/solenoid,it's stuck open,can't complete test for emissions,a tsb is out there for 99 -07 dirt clogging that valve making it unoperable

[Kristen]

Thanks 

[Kristen]

Would something like this cause the thermostat to fluctuate 

[riverbanks]

Stat is stat,new one, correct?what are you seeing,gage fluctuating?,no heat?rich fuel smell out of exhaust?

[Kristen]

No gas smell. Since my husband can't look at the car till this weekend today when I was driving the car was stalling out and the temp gauge went flat but I turned the car on and it started up fine and drive fine unless I was at a stop the min I stared driving again the car was fine 

[riverbanks]

Coolant temp sensor runs the gauge and provide temp input to pcm,a failed one will throw a code,a flaky one could be your problem