Time Machine Fails Backup

I have updated my iMac to Mojave and Time Machine is now failing to complete and update. I have run first aid not my time machine and my main hard drive and both pass. I connected a different HD to use as Time machine and have the same problem. My wife iMac is doing the same thing so it looks like it could be a bug with the new OS. Has anyone else run into this problem? I installed an update to App Cleaner & Uninstaller which may have contributed to the problem but I deleted the app and reinstalled Mojave and still have the same problem. The backup runs but fails after about 650 MB are completed then restart on the next hourly automatic backup only to fail again and again. Preparing backup seem to take a very long time (over 30 minutes) before the backup begins.