I have a PROCOM Model ML 300TBA, two years old. Lights fine, pilot stays lit fine, flame comes on fine. When I go back the next morning I am finding the unit is completely out. No flame, no pilot. I till light fine again. Any ideas?

Try a Honeywell universal thermocouple .

So we are talking about the same won't stay lit I will describe both ways. One is when you lift your fingers off the knob it goes out that instant which is probably the thermocouple needs replaced,look to make sure the tip of the thermocouple is in the flame of the pilot light first. The secound version of it is it goes out in a few minutes up to a few hours.The second version is most common.There is a part called a Thermocouple that is mounted next to the pilot light, it is around 3/16 of a inch diameter 1 to 2 inches long and should be mounted so the tip is in the flame of the pilot light. This is what tells the gas valve the pilot light is burning. If any dust ,cat hair ,dog fur what have you gets around the base of the hole the gas from this pilot light comes out of it will bend the gas, which move the flame so it does not hit the thermocouple like it should, this is below the flame so what ever is in there diverting the flame dose not burn up.A draft from a fan or door could do it also.I have used a procom ventless heater as my only heat for 7 years and have to blow it out twice a year. I bought a new thermocouple at the same time I bought the heater to have in the closet in case it went bad some night, it is still hanging there .Because it sucks air in from the bottom past the flame and out the top everything in the air gets burnt into a fine ash dust. So if you use compressed air to blow out your unit you might want to hold a wet towel up in front of it hoping a lot of that white powder dust will stick to it instead of flying around the room . My flame always looks so much better ,less orange and more blue after this .

Make sure the gas valve is on. The thermocouple may need to be cleaned in order for the pilot to stay lit.

Hit the flame sensor next to the pilot with sandpaper. If it still doesn't work replace it.

It sounds like you need to clean the contact on the thermocouple.

Here is a link to the service instruction of how to do that.

http://www.allpartsinc.com/mmAllParts/Others/Reseating%20Instructions.pdf

If pilot lights and flame is robust and clear blue, and pilot goes out later, then some suspects can be excluded: air in gas line, low gas pressure, thermocouple is overtightened (finger-tight + 1/4 turn), thermocouple not positioned correctly in the flame, dirty pilot tube.
If flame is yellowish and small: then clean pilot tube and combustion parts.
http://waterheatertimer.org/How-to-install-gas-water-heater.html#clean
Remaining suspects can include: air in gas line, low gas pressure, bad gas control valve, tight house so household vent fans pull air down water heater vent that blows out pilot, hot attic with inadequate ventilation, water dripping down vent.

Open following links to read troubleshoot resources.
http://waterheatertimer.org/pdf/Pilot-light-will-not-stay-lit.pdf
http://waterheatertimer.org/How-to-install-gas-water-heater.html#troubleshoot

You could try cleaning up the thermocouple with steel wool. Some manufacturers call this a flame generator as well. Any how make sure it is clean and positioned in the top center of the pilot light for best signal. This is only speculation as these are common problems that will arise on heaters in general. This also turns the gas to the burners and pilot light so thats why to start here first. Hope this helps you out.

all the solutions quoted have nothing to do with my problem.
if any of them applied, it wouldn't be a problem with 2 new heaters, as well as the 3 year old heater.
the flame of the pilot jumps away from the orifice after varying periods of time.

sounds like your thermacouplar is going out and needs replaced. Sometimes these will act like they are working fine , but when the pilot stays on for awhile and later goes out it is a sure sign that it is bad

If the burners do not fully ignite, the sensor will shut the furnace down. The lack of sufficient burner flame is a gas valve or gas pressure problem within the valve.
Replacing the gas valve with the original one may get you going. I surely suggest a professional service company for safety reasons and a thorough inspection. A 20 year old furnace is past it's normal life span.