1. Practise solving every F2L case with eyes closed until you can do them all no problem.
2. As above, but now find another pair first, and track that second pair mentally with your eyes closed while solving the first pair. (What you are practising with this is two things - both tracking how the second pieces move around the cube, and solving the first pair without mentally tracking it. With exercise #1 above, the temptation is still to think about the pieces being solved rather than solving the case purely from muscle memory.)
I tried for a long time with metronome and it was never comfortable for me. Solving to a fixed beat just destroyed my muscle memory recall.