I had an occasional problem like you. I replaced the Trim Tilt Relays. There are two in the fuse box and I think that from memory both are the same, but check first. If they are the same try swapping them over and see if it works. I bought them on Ebay for about a £5er each, think Evinrude wanted about £40 for one. There is a bypass on the valve if you want to do it manually, slowly back out the screw till things start to move.
Looks Slow but is Fast
Member of the ebay Blue RIB cover club.