Investigate temperature anomalies

[brettski74]

Temperature readings on an unpowered heating plate seem relatively accurate and around room temperature (eg. 24 celsius), but once power is applied, temperature readings initially go down substantially - as low as perhaps 10 celsius. They then seem to lag far behind the actual hot plate temperature resulting in the hotplate far exceeding the desired maximum temperature. Need to investigate:

1. Why temperature goes down when hotplate power is applied.
2. Whether temperature lag is due to some problem or simply the difference in temperature between the centre and edge of the hot plate.