hubs can become ovalized, causing the chain to have sections of strong tension and weak tension, if that is the case for you, backpedal to find the spot of less tension and adjust the wheel there
so long as you don't overdo it, your chain will stay tight
another possibility is that somehow the wheel is slipping forward, causing your chain to lose tension. this can be resolved using more torque on the bolts, or adding washers between the bolts and the frame dropouts
a third possibility is a bent or damaged tooth on your sprocket (or less likely on your rear cog/freewheel). backpedal and visually inspect the sprocket for any teeth that deviate from normal positions, keeping in mind some teeth angle differently on some sprockets, which can be normal. once found, you can potentially bend the tooth back. worst case, replace your sprocket (not your whole crank).
I suppose a 4th possibility is a bad chain, like a twisted or frozen link. visually inspect the chain as you pedal or backpedal, and observe how it goes over the sprocket or freewheel/cog, look for any deviation from the norm.
TLDR take it to a fucking bike shop