MacOS – Mail App keeps hanging (not crashing) on Yosemite 10.10.3

Mail runs fine for a few hours, then hangs. I must force quit, then reopen. It runs fine for another few hours then crashes.

This is a fresh install of Yosemite, and I rebuilt the index and tried deleting things people suggest I delete in the library. It redownloaded all the messages already.

Here's the first bit of crash report, which is Greek to me:

Date/Time:       2015-06-10 19:20:29 -0700
OS Version:      10.10.3 (Build 14D136)
Architecture:    x86_64
Report Version:  21

Command:         Mail
Path:            /Applications/Mail.app/Contents/MacOS/Mail
Version:         8.2 (2098)
Build Version:   1
Project Name:    Mail
Source Version:  2098000000000000
Parent:          launchd [1]
PID:             24057

Event:           hang
Duration:        3.80s (process was unresponsive for 3984 seconds before sampling)
Steps:           39 (100ms sampling interval)

Hardware model:  MacPro3,1
Active cpus:     8

Fan speed:       586 rpm

--------------------------------------------------
Timeline format: stacks are sorted chronologically
Use -i and -heavy to re-report with count sorting
--------------------------------------------------

Heaviest stack for the main thread of the target process:
  39  start + 1 (libdyld.dylib + 13769) [0x7fff8a4c45c9]
  39  NSApplicationMain + 1832 (AppKit + 13140) [0x7fff8fadd354]
  39  -[NSApplication run] + 594 (AppKit + 551923) [0x7fff8fb60bf3]
  39  -[NSApplication nextEventMatchingMask:untilDate:inMode:dequeue:] + 346 (AppKit + 593768) [0x7fff8fb6af68]
  39  _DPSNextEvent + 978 (AppKit + 596411) [0x7fff8fb6b9bb]
  39  _BlockUntilNextEventMatchingListInModeWithFilter + 71 (HIToolbox + 205099) [0x7fff8c4bf12b]
  39  ReceiveNextEventCommon + 431 (HIToolbox + 205546) [0x7fff8c4bf2ea]
  39  RunCurrentEventLoopInMode + 235 (HIToolbox + 206191) [0x7fff8c4bf56f]
  39  CFRunLoopRunSpecific + 328 (CoreFoundation + 465912) [0x7fff88299bf8]
  39  __CFRunLoopDoObservers + 368 (CoreFoundation + 524416) [0x7fff882a8080]
  39  __CFRUNLOOP_IS_CALLING_OUT_TO_AN_OBSERVER_CALLBACK_FUNCTION__ + 23 (CoreFoundation + 524583) [0x7fff882a8127]
  39  CA::Transaction::observer_callback(__CFRunLoopObserver*, unsigned long, void*) + 71 (QuartzCore + 118553) [0x7fff860fef19]
  39  CA::Transaction::commit() + 390 (QuartzCore + 58286) [0x7fff860f03ae]
  39  CA::Context::commit_transaction(CA::Transaction*) + 242 (QuartzCore + 58898) [0x7fff860f0612]
  39  CA::Layer::layout_and_display_if_needed(CA::Transaction*) + 35 (QuartzCore + 61057) [0x7fff860f0e81]
  39  CA::Layer::display_if_needed(CA::Transaction*) + 603 (QuartzCore + 63485) [0x7fff860f17fd]
  39  -[_NSViewBackingLayer display] + 834 (AppKit + 401702) [0x7fff8fb3c126]
  39  _NSBackingLayerDisplay + 617 (AppKit + 415019) [0x7fff8fb3f52b]
  39  CA::Layer::display_() + 1546 (QuartzCore + 70716) [0x7fff860f343c]
  39  x_blame_allocations + 81 (QuartzCore + 120468) [0x7fff860ff694]
  39  ___ZN2CA5Layer8display_Ev_block_invoke + 59 (QuartzCore + 120535) [0x7fff860ff6d7]
  39  CABackingStoreUpdate_ + 127 (QuartzCore + 120668) [0x7fff860ff75c]
  39  __psynch_cvwait + 10 (libsystem_kernel.dylib + 90422) [0x7fff8293c136]
 *39  psynch_cvcontinue + 0 (pthread + 26908) [0xffffff7f80ed891c]


Process:         Mail [24057]
Path:            /Applications/Mail.app/Contents/MacOS/Mail
Architecture:    x86_64
Parent:          launchd [1]
UID:             501
Sudden Term:     Dirty
Task size:       57324 pages (+4)
CPU Time:        0.074s
Note:            Unresponsive for 3984 seconds before sampling
Note:            3 idle work queue threads omitted