Thank you everyone for the great advice, I knew not to mess around with the natural gas so I called a plumber and my natural gas company.
The gas company inspected the meter and the lines and indicated there were no leaks or degradations in the line, even with a pressure test. The gas company upgraded my street valve and my house for good measure and the gas supply has never been better to every appliance. The couldn't figure out why I wasn't getting gas out to the heater and suggested that the line might have been pinched somewhere (only 4 years old and I had my doubts). Gas line is about 70ft run from the meter under earth and some concrete. The plumber was going to return the next day after the natural gas company had done their due diligence. On a hunch after finding water in that drip leg, I decided to shut the gas off totally to the back yard as it was on a separate valve from the house. So I opened up the drip leg to find no more water in there and connected my shop vac to the line. Within about 20 seconds I heard a huge glug of water (about a cup) come through and into the shop vac and then no more water, held it on there for a while to make sure nothing else was there. Made sure the shut offs were in place at the heater and then turned the gas back on, went back to the heater and BAM, an absolutely ton of gas was coming out the line at the heater as it should have been.
We had ALOT of driving rain for the past 70 days in the south and I'm wondering if water worked it's way from the heater exhaust somehow or if a malfunctioning heater shoved water back in the line?
In any case, I reconnected the heater and it fired up on the first try. Turned it off and on a few more times and it ignited with no issue every time. So I put the spa on and left it to heat. Came back an hour later to find the spa at full temp but the Service Heater light was on (NOOO!). So I opened the heater and looked at the Fenwal and it was flashing two lights. At this point, I decided to try the new grounding wire fix where you attach the wired to the blower top and below the blower as diagrammed by Pentair. First try after I connected the new ground wire, it fired and stay on, and over the next hour as it called for heat it would turn on with no issues. Just as we're about done the dreaded Service Heater light comes on again, even with the new ground fix. At this point, I shut everything off and called it a night. I canceled the plumber because he was already out once and identified the same no-gas issue there wasn't any else he was going to do for me outside of suggest running a new gas line which I don't need. Also I checked the drip leg several more times for water and its all bone dry.
I knew I was going to have a beautiful day in the south today so I had programmed the Easy Touch to turn the heat on for the pool. I woke up to the pool and the heater running perfectly. The pool heater ran all day flawlessly heating the pool to about 90 degrees. Once the pool reached the 90 degree mark, the heater turned off as expected. When it turned on again to keep the temperature up, the Service Heater light comes on with the same Fenwal 2 Flashes, no call for heat (no other sensor lights on the heater control panel) I suspect that if I leave it off for the night and it starts again tomorrow it will ignite and run fine from a cold (dry?) start.
So now I'm kind of back to where I was in November. The heater has gas, the igniter works great, and after a period of drying out or non-use it fires up first time every time. But after it gets to temperature it doesn't want to re-ignite, or if it does, its not reliable, even with a new ground fix. My pool company was waiting for me to figure out the gas issue before they returned but I am literally at a loss for what else this could be so I'll be calling them back over the next week. The heater and all of it's sensors clearly work and they can run for 12 hours+, but they don't want to reignite reliably after the system has been running for an extended period of time.
I think the worst part of all of this is I'll probably never have piece of mind that MasterTemp 400k will be reliable, but conversely I never had a single problem with it for the first 4 years of its life.
So guys.... any other special tricks up your sleeve?