Possible Control board problem

you might want to check where the thermometer is located on older models it can be knocked out of position perhaps to close to the roof of the oven which gives false heat reading

If you preset to 450F what does the oven stop at?
Do you have an oven thermometer that you can use to compare against the reading (327F) that the oven gives you?
It appears to me that the oven thinks its hot enough to stop heating. Could be a temperature reading problem, a comparison error, a display error or a calibration error.
Is there another reboot process other than unplugging?
How old is the appliance?

is this 400c or 400f

Your oven sensor has gone bad and telling the control board/thermostat to keep heating. Find the model number on the range and go to a parts distributor like repairclinic.com or apdepot.com and order you sensor. This is an easy repair.

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Read this........http://www.fixya.com/support/r4511800-oven_problems_electric_oven_heating

If you are monitoring the oven temperature with an analog style thermometer you may actually be ok, but seeing the thermometers slow reaction time.
Some newer ovens actually do pre-heat beep prior to reaching temp in an effort to conserve eelectricity.
A good home test is to try your oven at several different temps to see if there is any variation in internal temperature. Try it one day on 300, the next at 375 and another day at 450. Give your analog style thermometer time to catch up (usually just about 15 min) and leave it on for a half an hour to see if the temp maintains.
Also Beware of aluminum foil AND THE NEW OVEN LINER MATS as they can alter oven performance bay absorbing and blocking the ovens natural heat radiation and convection air flow currents that happen in all ovens, not just convection models.
Another interesting fact is that older ovens used a thermostat in them that operated much like your thermometer and was very slow to react making the oven typically about 100 degrees hotter than where the customer set it. This was just a fact of life before the advent of electronic temperature management and became noticeable when people began trying to bake older "hand me down" recipes in the newer ovens with less than stellar results.
You can mimic the older oven performance by preheating the oven about a hundred degrees higher than called for and after it reaches temp, re-set it to the correct temperature and put the items in right away.
If you find your temperatures fluctuating ask your servicer to replace first the temp sensor and as a last resort the control board as this part is usually pretty expensive.
Good Luck with this!

it is the bake relay sticking--on the eletronic oven control board.i tried to buy the relays but,no luck.must buy the eoc board-great engineering

The Taylor oven thermometer you bought is decent at best. It is most likely the sensor inside the oven. Has your clock displayed any fault codes? F, followed by a number? Changing the sensor will probably correct the problem.

Jim,
A couple of questions... What makes you think the oven is off that far? If you have a set on the shelf dial type thermometer, throw it away. They are never accurate and generally read 50+ degree too low. I recommend the "biscuit test". Which is simply a cheap can of "hit on the edge of a counter top to open" biscuits and see how they bake with the instructions on the can. Make sure you allow the oven to preheat for 20-30 minutes (always but especially for this test).
To change the oven setting press BAKE, then run the temp up to 550 then immediately press and hold the bake pad for 10-20 seconds. The display should read 0 (zero) you can then use the up down arrow pads to adjust, hit bake pad again and then "clear" when done to lock it in but, you can only go up or down 35 degrees. If it is truly off by more than that you would need to change the main oven control board and oven sensor.