Thanks all for your feed back.
I installed the cruise control myself a good many years ago. and been working fine up until now. The guys at MC Cruise control have been very helpful, but still the issues continues.
Strange thing was it was working then slowly began not holding speed over a run, which makes me think more a vacuum issue than speed sensor, but will continue looking at this.
It passes all the trouble shooting checks of switches, solenoids and cables, also calibration is fine is set to 4 which is as per manual.
I was thinking the vacuum hose may have a developed a hole and will be looking at this over the weekend hopefully.
If not will go back to basis and check every install step.