How hot are you asking the tankless to be? It may be that if you have the set temp way high that when mixed with the cold at the shower, the hot volume isn't enough to keep the thing firing (it does have a minimum flow rate). Can you try lowering it to say maybe 45-48 degrees C which would mean you'd be using almost all hot in a shower and see if it makes a difference? Really low-flow showerheads may be to low to trigger the thing.
The alternative is to chuck the thing and go with a traditional tank or use it to fill an indirect and add a recirculation pump to keep the tank hot. That would add considerably to the cost. My very limited experience with tankless has led me to discount them as an option for my lifestyle. They have their good and bad points. You should be able to make yours work better than it is, though.