Water Heaters - Intermittent hot water issue

What we have is an intermittent hot water issue. Sometimes we will have hot water after the dishwasher has run and someone has taken a shower already. Then other times when no one has used any hot water and the first person to take a shower has cold water. Any ideas?
thanks

next time you have that cold water situation go to the HWH and turn the temp dial CCW to get it to come on.keep turning it till the burner goes on.if the water is stone cold in the heater your gas valve thermostat is shot.a 12 o'clock position on the heater temp dial should give you hot water that needs some cold to temper it during a shower.pilot is consistanly on you haven't had to relight it?

Water seems to be lukewarm now with no hot water at all. Burner is not coming on after turning the temp dial up. Although the person who checked it at home says that the pilot is not lighting at all but that would not produce lukewarm water would it?
thanks

What kind of hot water tank are we dealing with here? Does it have a a continuous standing pilot, an electric start for a pilot light which then fires the main burner, or an electric start for the main burner with no pilot? Is this a direct vented tank or power vented unit? Make and model would be helpful.

AO Smith, FPSH 40 252

Electronic igniter rather than standing pilot. I'm going to guess elect start for burner with no pilot.
I spoke with parts/tech at AO Smith and one person said it needs a new smart valve/electronic control and another said it was probably the hot surface igniter. Igniter is $138 and smart valve is $230. I don't want to throw money at it and end up needing both items, so I'm going to try to clean the igniter and see if that takes care of it. If I end up buying both, it won't bee too far from the cost of a new heater. Any ideas?
thanks

As near as I can tell it looks like the thermostat is integrated into the smart valve, so the next time this happens unplug the HSI and see if you are getting any voltage. That may help determine where the problem is.

It appears that on those units with the elect smart valve, the green light flashes a certain number of times to display an error code. My unit flashed 6 times and that means that one issue could be that the flame sensor rod is malfunctioning and causing the smart valve to cycle 3 times to keep the burner going. If it does not sense a lit burner after 3 tries, it shuts down for an hr. The solution for that is to either clean with steel wool or emory cloth the surface of the metal sensor rod, or replace the igniter assy.

The other thing the 6 flash code could indicate is a faulty gas pressure regulator inside the smart valve which would then need replacement of the smart valve.

I cleaned the sensor rod and it seems to be working fine. They recommend cleaning at least once a year.

thanks