Most likly a defective "solder connection" on the circuit board. The problem will get worse as time goes by and could possibly damage some "parts" in the affected circuit. I would suspect "power supply or high voltage circuit". As far as a "reasonable charge" in your area, I don't have a clue! Shops in different areas charge different prices. You might call around, give them the model number, the symptoms and ask if they might give you "an over the phone estimate". Good luck, pw

Okay...it looks like he replaced the old mechanism (the type I was describing with a float ball) with a newer model, which is a good thing, and that you also have a new flapper (the red thing). The height of that gray and black portion of the valve can be adjusted to get the water to come up to the correct level in the tank, and it's possible your husband did not do that correctly when he installed it. Do you see that black plastic vertical rod that connects the top of the valve to the black and gray part that floats? If you rotate that, it should adjust the position of the float up and down. You will likely want that floating part to be positioned lower, which you can do by trial and error. Here's is how to do it: turn the water back on and let the tank fill and see where the water stops in relation to the water line. If it is higher than the water line, turn the top of the rod a couple of turns (probably counterclockwise) so that the float goes deeper into the water. Then flush again and see where the water rises to. It may take a couple of these adjustments to get it to stop right at the water line. After you do that, post back and tell me what's happening and whether it's still hissing or running. Then I will give you some more advice. (I should note that if you can't get the water to the correct height with the adjustment rod, or you turn it as far as you can and it won't go any further, there is another adjustment that can be made to lower the entire valve, which also may not have been done right when it was installed. This is a bit harder to do and you may want to task your husband-plumber with this when he returns.) Below is a link to a 6 minute video of how these valves are installed and adjusted. Most of it shows how to make this installation from scratch, which has already been done, but watching this will give you a good idea of how everything works and how the adjustments are done: Toilet Valve replacement As for the water level in the bowl, if it is not a problem with any of the other toilets, then it is unlikely to be a root problem. It's either: 1) that this is the way this old toilet works now or was designed to work originally, or 2) that there is some type of partial clog nearby that only involves this toilet. (I once had a problem like that with a toilet that previously had been efficient and it turned out someone had flushed a popsicle stick down the toilet and it was partially blocking the outlet.)

So the problem seems to be at the tower? try this pull power plug from tower only, then while it is out hold in the button for 20 secs or so. then plug power cable back and start see if this helps, sometimes it builds up an overload of power.

In sandy and calcareous soils, foliar applied fertilizers are 4 to 20 times more effective when compared with soil applied fertilizers. (Lingle & Holmberg, 1956.) https://citrusagents.ifas.ufl.edu/archived_presentations/fall_prg2010/Citrus%20Agents%20Web%20Site%20-%20Nutritional.pdf Citrus foliar fertilization has been an industry standard practice in citrus groves for over 50 years. https://crec.ifas.ufl.edu/extension/trade_journals/2014/2014_April_citriculture.pdf Where soil PH is too high or too low, resulting in 'lock up' of nutrients (frequently micronutrients) in the soil, foliar application of those nutrients will ensure rapid response in the trees and prevent growth from being restricted. Effect of irrigation water acidification and soil pH on citrus nutrient availability https://citrusresearch.ifas.ufl.edu/wp-content/uploads/2019/09/7.-Morgan-Expo-2019.pdf Four winds growers sells over 50 varieties of citrus nationwide. Here is what they say about misting, "In winter months, heated rooms may need additional humidity. Placing the pot on pebbles in a saucer will elevate the tree above the drainage area, and improve airflow and humidity for citrus. Misting citrus foliage with a simple spray bottle is another way to help citrus cope with insufficient indoor humidity in winter." If you have new leaves and new shoots it is really beneficial to mist those. Because of the VPD they loose moisture much faster than mature leaves. Flowers are also very susceptible to losing moisture and misting can help them set fruit.