The biggest problem I see is how you're reading in the data in the event handler. The event handler gives you back the data from the sensor input in the "e" variable, but you just ignore it, and then poll the device for data again. This is going to take some time, and is probably where your problems are coming from.
For example, this:
ph = 7 - (2.5 - phidgetIFK.sensors(1).Value / 200) / (0.257179 + 0.000941468 * temp)
Should be more like this:
if (e.index == 1) ph = 7 - (2.5 - e.Value / 200) / (0.257179 + 0.000941468 * temp)
This code simply checks to make sure that it was sensor 1 that triggered the event, and then reads the data that the event handler was given. Make sense?
You also shouldn't be setting the sensitivity of your input every time your event handler is run. Set it once in your attach event, and you will be good.
It may also be a good idea not to poll the thermocouple for a temperature every time your sensor change event runs.