I have had a mason alarm (with no issues) fitted to Trjegul for the last 3 years and have just started to encounter a strange problem. I was driving along my usual to work route and the gauge kept climbing up to the red so I hobbled in to work and at standstill with the heater on it dropped a little. Then I noticed that if I operated the electric window, the guage dropped suddenly, ditto with the headlight switch, and IIRC the wipers

Has anyone ever had a similar problem or any ideas on what it could be and how to resolve it? The mason-modded gauge has been a boon to me- saved my bacon when I had a hose go, so I want to get back to where I was before this! I am a complete non-expert on matters electrical btw!
Many thanks in advance
Zin