I'm seeing this behavior tonight in Reaper 5.5RC5/64 on Mac OS 10.11.6.
MacBook Pro
2.8GHz Intel Core i7
16GB 1600MHz DDR3 RAM
NVIDIA GeForce GT 750M 2GB video
Upping the Render-ahead setting to 600ms does improve things a bit but it still happens.
VST and AU versions are fine, but then I lose the benefit of track number/name/select chasing
