Check out FAQs at http://www.valvetronix.net/valvetronix/ all you need to know is there sounds like the usual jack plug problem

Nasty business at the end of a note is often the noise gate. Turn it off and see it that helps.
I'm assuming the clean note sounds fine for the duration and just does this at the end.

I have tried cleaning all the jack plug sockets with contact cleaner but that didn't work unless maybe I need to repeat the process several times.

Badly oxidised sockets may need cleaning up to 9-10 times. Pay special attention to the switching jacks - headphone, FX loop, & Line outs (but especially the first 2). These can stick, preventing a clean contact and deteriorating the signal thus resulting in crackles, hiss, distortion etc. This is THE single biggest issue with Valvetronix amps, but its also the easiest problem to fix - just needs some patience.

The other thing to try that's free, quick & easy is an amp reset back to factory settings which will usually fix any glitches caused by software filepath errors.

It's all explained here:

http://www.valvetronix.net/forums/maintenance-tips-on-valvetronix-amps-t141.html

Try your power-selector in different positions to see if its the same. If it's only in one or two positions, if you can, try to squeeze in some cleaner in the power-selector at the rear of the amp.

The other thing is to remember that it might not actually be the amp! Spray switch cleaner into your guitars input, any external FX boxes, and to even try a different cable - even minor issues with all of these things could give you similar symptoms, so its important to take a holistic view - even if you have no problems with another amp, remember that modeling amps are particularly sensitive to things that give rise to even mild signal degradation issues.

Rich

I cleaned all the jack sockets again last night with servisol super 10 contact cleaner as per voxmans advice and today the amp is working perfectly it sounds absolutely sweet again.
Its amazing what that stuff can do.

Thanks for all the advice. Yet again I have been saved from a possibly expensive trip to the amp repair person.