Gas valve or flame sensor Extra - no way it's air in the lines. I would clean and then if that doesn't work replace the flame sensor first. Then get a gas valve if that doesn't fix it.
These things aren't rocket science, they are about as simple as it gets.
There is no need to replace your hot water heater. People always replace stuff that doesn't need it.
The plumbers make all their money doing this, my neighbour just paid over $1000 because his blower motor quit in his furnace. We built at the same time so have identical furnaces. The furnace tech sold him a new flame sensor and "ball bearing" blower motor (which was gone). He sold him on how much better ball bearing motors are than bushing ones and he ended up spending over $500 alone on the motor.
Mine went 2 weeks later and I went to Amre supply and picked one up for under $200.00. It took me about an hour to swap it out and the things been running fine since.
Don't get hosed, just fix it yourself.
2017 VW Jetta 1.4T (winter car)
2010 F-350 Nortruck Tool Shed
2007 BMW 5 (Wifey's)
2004 Audi A4 1.8T 6spd Stage III w/ Meth
1985 F-250 6.9L IDI w/ATS turbo