Hello!
I do not have enough element to tell the solution, but...
I do not think that your problem is coming from a lack of O2. The %age of O2 being (+/-) the same whatever the altitude; only the Atm. Pres changes.
Not enough pressure in your gas tank will not help for sure. Above all if it's cold. Low Temp = Low Press. = weak blow.
At 5000 ft theatmosphéric pressure is lower, but good enough to work out your Steam Plant.
Most of the time,
if the boiler needs more than 10 min to hardly reach 25/30 psi, it is due to the jet partly plugged in by debris. It happens more often that we believe.
Fortunately, we have here , people steaming the same plant as yours, in the Alps , at 1600 m (+/- 5300 ft) by 10°C , with no problem.
The temperature may be one, though. Yesterday, I sailed (at sea level) with a ambiant temp of 16°C. My burner supplied by 100% Butane gas. The pressure in my gas tank was too low (0.2 bar) to have a good flare. By using then, 30/70 mixture, the burner spitted as a Saudi flare in a good day. So temp may be the issue more than the altitude.
Sailing below 7°C is a problem in term of positive burner gas supply, but can be solve by using liquide phase.
The other option is to insert a thermic bridge between the gas tank and the burner. Just a piece of brass, that you can remove for hot days.
I would conclude by saying that the combination of a plugged jet and a low temp may give you hassles.But not the %age of O2.
Remember that the optimum temp for steaming is when the beer is at the right temp!
(except for freezed Budweiser drinkers!
)
Concerning your engine, I have the feeling that the difficulty you have to restart it after a STOP is due to the synchronisation of 1/4 turn valve opening and the slide valve movement.
I think that the main steam valve opens (just a crack is enough) to pressure up the slide valve sticking it against the wall. No pressure on the other side as you stopped.
Then the servo is too weak to move this slide valve enough to allow some steam to pass and allowing the machine to run.
Maybe something to reconsider. Use one servo (stronger) for each function.
Sorry to be so talkative , but I do not see nothing wrong with your plant. I don't know if I gave the right solution, but I would look in this direction first.
Thanks for reading and sorry to be so long. Hoping you understand my "globish".
Regards.
MODELISME BATEAU A VAPEUR