What causes engines to diesel/run on when you shut em off?

I have this problem with my Buick at the moment. I'm not sure if it's running a little too hot, or if the timing isn't quite right, or if the fuel mixture is too weak. The engine was completely rebuilt in 2013, at one point I had twin fire plugs in it, but they were shorter than the standard plug. I have since put the correct plugs in, this has helped a lot with the tick over, the car now runs really nice, fires up straight away, but once warmed up after a short drive, it suffers with the run on when turned off. I checked my plugs the other day, and they are a light grey, so I would ***ume my mixture is too weak. I checked my thermostat in a pot of water last night, it opens fully around the right temperature, but the opening seems a bit small considering the size of the thermostat housing. Is it worth removing the thermostat completely during the summer?