My LP oven will ignite for a couple seconds then cut out. If set to 500 degrees it will reach 350 in about 30 minutes but never goes above 400 and that's if I turn it off then on again a couple times. It also won't go into clean mode. When I put in clean mode I hear a very faint click but nothing happens.

what is your oven if is gas or electric, if is gas easy to preheat by using lower heating element, if electric? that your unit have convection? or not? coz some probelm of the oven is, 1 if the oven is no convection or your oven is single heating in upper element,if your oven have a timer and cut off in 15minute? dont use timer,use a manual timer by it your self,if you want to reach the degrees that you want check your oven if der a lower and upper heating element with convection function,that the one is the most easy way to reach the degree that you want to reach.

the previous owner was on lp gas, you are on natural gas. the orfice ratings are different. call a professional to convert the range back to natural gas.

Are you checking with a store bought thermometer or your recipe. If it is your recipe then ur thermostat might need a calibration or need to be changed completely if it is really off by more than 100 degrees. If it is 99 or less a simple calibration may just do it. If you have a steady hand. With your thermometer inside the oven and the door closed set at 350. Then take the oven knob off and take a small flat screw driver (really small). And insert it in the valve part where the knob was. With a flash light u can see the small screw. Turn it a hair at a time (1-2mm). Wait 20 minutes and see if the temp does rise. If not turn another hair. Repeat until 350 is reached. You must wait the 20-30 minutes for it to reach the temp. If all else fails changing the thermostat would be next. I do not recommend u do this alone. approx cost is: $400 - 500 front access, 500 - 600 rear access Good luck.

how do you know it is dropping 100 degrees ?
If the set temperature is 350-400 the heaters will work untill the thermostat will read that the temperature is reached - after that the heaters will shut down and the temperature will drop...and after few minutes when the temperature goes below thw set point (in this case 350-400) the heaters will turn on again - this will happen for the duration of the timing you have selected (example 45 minutes)

faulty switch, change the switch.

Not sure on your climate.
But a Holland grill cooks cooler in the coller months and Hotter in the hotter months. This type of grill doesn't reach a very high setting approx: 400 degrees. It might reach higher temperatures in the hotter weather 450-475 degrees.

Also, it has to shut down the same way every time you use your grill.

Here is a Tip to fix a grill that not getting hot:
1. Turn the gas valve knob off. then close LP tank valve knob.
2. Unscrew the plastic hose coupling knob from the LP tank. You may hear a slight "hissing" sound as pressure is released this is normal.
3.Wait a minute then screw the coupling knob back on to LP Tank.
4.With lid open.slowly open LP tank valve.
5.Relight the grill the proper way by turning gas valve knob to ON position while turning the igniter knob at the same time.
6.After you are sure the grill is lit preheat the grill for 30 minutes.

This was called purging the line or releasing the air out of the line.

Another thing your orifice could be clogged with spiders or dirt. If it is LP gas check to make sure your hose is not kinked.

Hollands are known for the slow cooking process and a Healthy way of cooking!

You may not necessarily need a new control panel. Sometimes the oven temperature can be calibrated. Look in your owner's manual (if you have one) for calibration instructions. This will usually take care of the problem. If not, you may have to replace the oven sensor located inside the oven in the upper right or left hand corner of the back wall. Good luck.

sometimes when they get hot that is when they fail

Check the element for burns/wholes this was problem for our.  Easy fix and part is about 35 to 40 dollars.