Occasionally while flying the subject aircraft, sometimes after landing and sometimes holding and ready to proceed onto the assigned runway for takeoff I loose the control of one engine when I advance the throttle. I can

This has happened to me when I accidently touch one of MSFS default keys for left or right throttle... from that time onward I've only got single engine control. Only fix (I found) is to restart MSFS. Maybe this is your problem.

If you have a registered vesion of FSUIPC there is an option on one of the pages to set up a keystroke to sync all engines.

On your keyboard select 'shift+1 and then 2 very quickly. It happens sometimes.

The other thing to do is remove the "E" key assignment from FS key assignments menu.