It sounds like you might be experiencing an air intrusion or fluid flow problem in your Hyster H40H hydraulic system. Steering and lifting could be the functions that are most likely to be impaired. Although you have already changed the filter and cleaned the strainer, it might be that the air is entering the system via a loose or damaged fitting, a crack in the suction line, or a failing pump seal. One of the effective ways could be to check all suction lines for any sings of wear or cracks that might allow air in. Also, inspect the hydraulic pump in case of wear or damage, particularly the seals that may sometimes fail and let air in while under load. A careful examination of the hoses, fittings, and seals around the pump, and confirming if the reservoir is not contaminated or overfilled, can contribute to the recognition of the true cause. The fact that the disturbance is sometimes removed by just loosening the hose fitting, might also suggest that there could be a possible blockage in the return line or the relief valve might be faulty. A pressure gauge, if you have it, may be used to test the pump pressure to determine any abnormalities in the hydraulic system's performance.