My oil pressure gauge goes to "h" when I stop and the light of " check gauge" turn on. When I start running again, the gauge goes to the middle and the light of "check gauge turn off". What could be happening?

Did the mechanic drive the truck with a mechanical gauge to check the pressure ? The engine bearings or oil pump could be worn enough to allow pressure to drop at idle after it warms up.

Check oil level on dipstick. Check for loose connection on oil pressure sender unit. Have the oil pressure checked using a master gauge.

have a mechanic test the oil pressure with a mechanical gauge
it is more than likely that the electric sender unit has failed and is sticking internally
the tic noise could be a relay working from an intermittent power supply or bad ground

Okay What it means is that the oil in your car is either low in level or the oil pressure at low revolutions is low. I highly recommend you to stop driving the car when this happen because you can internally damage the engine with a low oil or a low pressure of oil. Check the oil level first..make sure level is fine and oil looks good. Black and burned oil will give you a low oil pressure as well. If it happen that your car need a oil change. Make sure you get it done and see if the problem goes away. Very rarely oils pressure sensor goes bad but if oil is new and level is fine you can take the oil pressure sensor and connect a tester for a oil pressure. If this is not under the manufacturer specifications.. oil pump is worn out and you have to replace it for a new one.
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Possible that the OIL filter is clogged and is reducing the pressure due to this when the engine is idling or in low RPM. So before you run the engine further and make it more damaging , it is worth to have the engine oil / oil filter changed. If there is still no result, check the oil pressure switch. If not the oil pump can be faulty. This needs a technical help and should be consulted.

start by checking the ground strap to the engine, and engine to body. The oil pressure sender unit may be bad. You may have to hook up an oil pressure gauge to the oil pressure sender port to verify proper oil pressure. Senders are cheap- I would just replace it and go from there.

You could try thicker oil or stop using synthetic or add some lucas oil treatment or change the oil. The sender could be bad too. A diag is in order because if there is no presure the engine will soon die.

hook up a mechanical gauge and verify the pressure at idle is correct, I suspect the sending unit is defective

First thing I'd check is the oil level, coolant level. The check gauges light is illuminating because on of the gauges is below a certain pre-programmed threshold. The light is very similar to the old idiot lights that came on when it was too late. Cars used to have gauges and in the earlly 90's the manufacturers figured out that the majority of owners did'nt even read the owners manuals, let alone know how to read a gauge. So they started putting in idiot lights that would come on when there was a problem, they figured even an idiot could see a red light on and know there was a problem. The only problem was that the pressure switches that turned the lights on were set very low. Take for example an oil pressure switch (OPS). The switch is designed to close and complete a path to ground for a light, the problem is it closes aroung 3psi. Normal oil pressure is around 30, at 3 psi the cam and crank bearings are starving for oil, by the time you see the light on it's too late, the damage has already been done to the engine. So the manufacturers started installing gauges and lights to cover all the bases. So if I had to take a guess, I'd say you have and oil pressure problem. Check the oil, if it's low figure out where it's going, if it's not then you may have a faulty sending unit, confirm this by installing a manual oil pressure gauge in it's place. If the manual gauge shows low oil pressure then there may be some debris/sludge floating around in the oil pan stopping up the pickup screen or it could be a faulty pump.

check the oil viscosity,it might be too old(driven for toomany miles)..

if thats not ityou might need to get the oil preasure lubricating pump checked,and id suggest you do it fast or theres a lot of engine wear and tear.