Inspiron 7520 15SE 2nd Display Issue

I've been running Windows 8.1 Pro since last year, but in the last month it has developed a problem. I frequently lose my 2nd display, an external monitor (VGA). I have to re-install the video drivers, but it never lasts beyond a few reboots before losing it again. The video drivers installed are 9NNDW_WN_13.151.0.0_A00 and KK6CK_WN_10.18.10.3277_A00. Any ideas what is causing this problem and how I can correct it?