Hotmetal I use a towel round the group often as it saves energy and warms up faster. If every so often you are getting lower temperatures, you may well have air entering/water leaking from the sealed brew circuit.
- 1 way valve
- Leaky top E61 valve
- Leaky expansion valve
A brew circuit/boiler descale might solve it…or you may need to replace a part/O ring.
test 1 way valve by leaving water inlet tube out of tank pointing upwards above height of machine,, see if the meniscus moves up…if it does, the one way valve is letting by.
Expansion valve is more difficult, as the water can easily evaporate away…but with machine in steady state, fine where expansion valve exits and make sure area is dry then regularily monitor. It’s the same issue with the top group vallve. Machine in steady state, dry hot portafilter, check after 30m…is it moist?