COLD WATER FOM HOT WATER TAP - GLOW WORK COMPACT ELECTRONIC?

Hello all,
I wonder if you can help?
My compact glow worm compact boiler is situated downstairs and my bathroom is upstairs.
When i turn on any of the hot water taps in the bathroom, the water is hot initially and than turns cold. If i turn the tap so that the water has a slow flow, the water becomes hot again.
Any ideas why this is happening. Any help would be much appreciated.
Thanks for listening!
Best wishes Philip