Intermittent segmentation fault from PyQt4?

Hey

My python 3.5 program encounters an intermittent segmentation fault approximately 30% of the times I run it. Nothing is changed between the runs, just start the program over. Through trials I've determined 3 calls to QLCDNumber.Display() to be the most likely cause of the faults. Replacing these with QLabel.setText() gives the same fault.

 Running the program in dbg gives the following output.

 gdb python3
GNU gdb (Ubuntu 7.11.1-0ubuntu1~16.5) 7.11.1
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Reading symbols from python3...(no debugging symbols found)...done.
(gdb) run GUI3.py
Starting program: /usr/bin/python3 GUI3.py
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
[New Thread 0x7ffff3ad0700 (LWP 26031)]
[New Thread 0x7ffff12cf700 (LWP 26032)]
[New Thread 0x7fffeeace700 (LWP 26033)]
[New Thread 0x7fffd3da2700 (LWP 26034)]
[New Thread 0x7fffd2042700 (LWP 26035)]
[New Thread 0x7fffd1841700 (LWP 26036)]
[New Thread 0x7fffd1040700 (LWP 26037)]
[New Thread 0x7fffbb952700 (LWP 26038)]
Run function needs to be implemented

Thread 1 "python3" received signal SIGSEGV, Segmentation fault.
QWidgetBackingStore::sync (this=0x1ec5d30) at painting/qbackingstore.cpp:1239
1239    painting/qbackingstore.cpp: No such file or directory.
(gdb) backtrace
#0  QWidgetBackingStore::sync (this=0x1ec5d30) at painting/qbackingstore.cpp:1239
#1  0x00007fffe0e11c50 in QWidgetPrivate::syncBackingStore (this=0x1907c20)
    at kernel/qwidget.cpp:1894
#2  0x00007fffe0e249b8 in QWidget::event (this=this@entry=0x1b3b380, event=event@entry=0x20bab70)
    at kernel/qwidget.cpp:8693
#3  0x00007fffe12089b3 in QMainWindow::event (this=0x1b3b380, event=0x20bab70)
    at widgets/qmainwindow.cpp:1478
#4  0x00007fffe1cfc28b in ?? ()
   from /usr/lib/python3/dist-packages/PyQt4/QtGui.cpython-35m-x86_64-linux-gnu.so
#5  0x00007fffe0dcdfdc in QApplicationPrivate::notify_helper (this=this@entry=0x18f9d70,
    receiver=receiver@entry=0x1b3b380, e=e@entry=0x20bab70) at kernel/qapplication.cpp:4570
#6  0x00007fffe0dd4f16 in QApplication::notify (this=0x17de970, receiver=0x1b3b380, e=0x20bab70)
    at kernel/qapplication.cpp:4356
#7  0x00007fffe1ebc0c6 in ?? ()
   from /usr/lib/python3/dist-packages/PyQt4/QtGui.cpython-35m-x86_64-linux-gnu.so
#8  0x00007fffe2eb390d in QCoreApplication::notifyInternal (this=0x17de970,
    receiver=receiver@entry=0x1b3b380, event=event@entry=0x20bab70)
    at kernel/qcoreapplication.cpp:955
#9  0x00007fffe2eb73c6 in QCoreApplication::sendEvent (event=0x20bab70, receiver=0x1b3b380)
    at ../../include/QtCore/../../src/corelib/kernel/qcoreapplication.h:231
#10 QCoreApplicationPrivate::sendPostedEvents (receiver=receiver@entry=0x0,
    event_type=event_type@entry=0, data=0x18fc490) at kernel/qcoreapplication.cpp:1579
#11 0x00007fffe2eb76a3 in QCoreApplication::sendPostedEvents (receiver=receiver@entry=0x0,
    event_type=event_type@entry=0) at kernel/qcoreapplication.cpp:1472
#12 0x00007fffe2ee413e in QCoreApplication::sendPostedEvents ()
    at ../../include/QtCore/../../src/corelib/kernel/qcoreapplication.h:236
#13 postEventSourceDispatch (s=0x1900aa0) at kernel/qeventdispatcher_glib.cpp:300
#14 0x00007fffe2a62197 in g_main_context_dispatch () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#15 0x00007fffe2a623f0 in ?? () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#16 0x00007fffe2a6249c in g_main_context_iteration () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#17 0x00007fffe2ee42ae in QEventDispatcherGlib::processEvents (this=0x17be390, flags=...)
    at kernel/qeventdispatcher_glib.cpp:450
#18 0x00007fffe0e7b616 in QGuiEventDispatcherGlib::processEvents (this=<optimised out>, flags=...)
    at kernel/qguieventdispatcher_glib.cpp:204
#19 0x00007fffe2eb218f in QEventLoop::processEvents (this=this@entry=0x7fffffffd7d0, flags=...)
    at kernel/qeventloop.cpp:149
#20 0x00007fffe2eb24f5 in QEventLoop::exec (this=this@entry=0x7fffffffd7d0, flags=...)
    at kernel/qeventloop.cpp:204
#21 0x00007fffe2eb84b9 in QCoreApplication::exec () at kernel/qcoreapplication.cpp:1227
#22 0x00007fffe1e6aa2b in ?? ()
   from /usr/lib/python3/dist-packages/PyQt4/QtGui.cpython-35m-x86_64-linux-gnu.so
#23 0x00000000004e9b7f in PyCFunction_Call ()
#24 0x00000000005372f4 in PyEval_EvalFrameEx ()
#25 0x0000000000540199 in ?? ()
#26 0x0000000000540e4f in PyEval_EvalCode ()
#27 0x000000000060c272 in ?? ()
#28 0x000000000060e71a in PyRun_FileExFlags ()
#29 0x000000000060ef0c in PyRun_SimpleFileExFlags ()
#30 0x000000000063fb26 in Py_Main ()
#31 0x00000000004cfeb1 in main ()