Don't understand how valves can be fitted any old how- they should have a central spigot keyed to fit the base in only one orientation. Loose or dirty contacts in the valve socket can cause the symptoms you describe.
There are at least two capacitors, C25 and C26 on the manufacturer's circuit diagram (
https://www.service-data.com/product...55/1369/m10955 ) which should be replaced if they haven't been already or they can cause expensive problems if they get too leaky (low insulation resistance). Others will likely also be bad, but will only affect performance rather than causing damage.