Single gas oven gas flame maintains heat until door is closed and then it goes very low.Any ideas?

If the oven will light and work with the door open but flames out with the door closed, you have the air input blocked somehow. I know some people line their ovens with aluminum foil, but you have to make sure that you have the air flow.

Opening and closing the door will reset the ignition sequence, do not do this repeatedly as you could cause a build-up of unburned gas in your heat exchanger and damage it. Instead verify that your ignitor is glowing, then see if the burner lights and make sure the flame carries all the way down to the bottom of the burner. If the flame stalls halfway or doesn't make it all the way to the bottom in 4 seconds after 3 attempts you will experience an ignition lockout, a safety feature built into your ignition module. If this is the case your burner needs to be cleaned and serviced, have a professional do it.
If the flame is reaching the bottom of the burner but still going out after about 4 seconds then you have a dirty/broken flame sensor. You can attempt to clean your flame sensor by removing the screw that holds it into the burner. With it removed it will look like a metal rod going through some ceramic, clean the metal rod part with a wire brush or emory cloth (fine sandpaper). Replace the flame rod and try again and see if the burner stays on this time. If it still goes out your burner needs to be serviced by a licensed technician.

cannot get the oven to heat up above 400f

Several things to check. First be sure the pilot light flame is directly striking the thermocouple. It is a pencil sized rod that is next to the pilot light. Sometimes people reduce the size of the pilot flame to save gas. You may need to increase the pilot flame. Follow the small gas line from the pilot flame back to the gas valve. There is a screw where the small pilot gas line goes into the valve. Remove the screw and then deep in the hole is a tiny screw that can only be reached by a tiny flat blade screw driver. Turn the screw counter clockwise to increase the pilot flame. The thermocouple needs to be red hot to work properly. The second item to check is to see if there are any breezes that occasionally blow through the kitchen. Sometimes the hood fan will draw air under the ovens and actually blow the pilot flame out. Same fix, increase the flame. You may also experience low gas pressure to the appliance during heavy usage. You would need a manometer gauge to check for proper gas pressure. In short, if the oven goes out during your busiest time when everything gas powered is on, the oven may be starved for gas.

replace the gas safety valve thermocouple,it has a thin coil of copper colored tubing going from the safety gas valve into the pilot light flame and as the flame heats the copper tube end it keeps the gas valve open to supply gas to the unit,they get old from being in a flame and cant signal the gas valve to stay open any longer,they just unscrew out of the gas valve and the opposite end is positioned in the tip of the pilot flame to heat it

Your door switch is not reading that the unit is closed

check your manual as some models that have a grill at the top requires the door to stay partially open.

replace the bake ignitor its getting weak

on the gas valve there is an adj. for the pilot size, maybe too small , would try increasing the pilot and see what happens.also the thermocouple might be on the way out ,you might want to replace that and try oper. go with thermocouple first, then try pilot size dont forget to vote tks. monomoy

your gas valve may be freeflowing you must have this checked right away