When a plug-in's User Presets are linked to ProgramChange messages, and Reaper plays, and an incoming ProgramChange causes a preset switch, Reaper freezes.
The sample project here:
https://app.box.com/s/rerokjeg4as46p7y7gp1
contains 2 tracks: one has a wavefile playing, and a ReaComp in an insert slot; ReaComp presets are linked to ProgramChanges on MIDI channel 1. The other track is recording MIDI (maybe you should check the selected MIDI input) and sends MIDI to the first track.
Press Play, and send some ProgramChange to Reaper on Channel 1 (low numbers, please check the number of entries in your presets list). Just a few messages, and Reaper will freeze (interestingly, the audio card buffer is always cleared in my case, I am always left with silence).
Freeze doesn't occur when switching in the ReaComp window presets list using the mouse on the presets ComboBox.
Reaper 4.402 x64
Windows 7 Home Premium 64bit
MOTU AudioExpress on USB
ASUS B121 w/ Intel Core i5-470UM, 4G RAM