Gearbox problems?

Looking for some inspiration! I have 3 westwood ride on mowers, when in drive all three sort of momentarily drop out of drive and back in. There's no noise with it, it's just a continual stop start effect that gets worse. I took the good back axle/gearbox out of my oldest one and refitted it to the newest, worked fine for a few weeks then started doing the same. I now have 2 or3 forward speeds only and they drop in and out of drive. The gearbox that I took out of the newest machine actually looks as if it had been refurbished, I'm wondering if it's actually not the box but something else, any ideas?