Telescope control

HELP! I used to have this all working but now it is playing about.

I have installed the latest Stellarium version. I have a Skywatcher EQ6 with version 3.27 software. I run Windows 7 and have a Prolific PL-2303 USB to serial adaptor.

The scope setup in Stellarium is this:

Com3
Localhost Port 10001
Skywatcher synscan v3 or greater

The problem? I only partially works. Sometimes pressing Alt+ 1 will move the scope, but not to where I have clicked the cursor to, just some where elsewhere. Then when clicking another object and Alt +1 it will move and return to the old object - very odd. Sometimes it will respond correctly to co-ordinate commands. However, usually after one move to a new object, it not respond again. Occasionally, stopping and re-starting the scope control may enable it to work - kind of! There seems to be no logic to this, as though in some way it is not calibrated.

I have tried installing a virtual serial port, but am not at all sure what settings to use. I've tried re-installing the Prolific driver, I've tried changing baud rate, all to no avail.

I am now totally frustrated, especially as I had it all working a while ago; can anyone throw some light on this please?

Phil