Empty the reservoir.
Then, in a jug, mix half a litre of lukewarm water and two/three heaped Tablespoons of Citric Acid. You can find it in supermarkets for about 80cents. When its fully mixed, pour it into the reservoir, and run about a third of the mixture through the brew head (into a large bowl), wih the shower screen and rubber seal removed.
Then switch it over to steam and let it run for about 5 minutes (i put a pyrex jug under the steam arm so the bench doesnt end up with a big puddle), then turn the machine off. Let it sit for 15 minutes.
Turn the machine back on, then run half of the remaining mixture through the brew head, and then switching the steam on for about another 5-6 minutes. Finally, if theres any of the mixture left, just run it through the brew head again.
Then, rinse out the reservoir, and run two full reservoir amounts (just normal, clean water) through the brew head. Refill the reservoir again with clean water and run steam through the steam wand for about 10 mins.
Reattach the rubber seal and shower screen, and pull some seasoning shots. Taste one, and if theres still a citric acid kind of taste, you need to flush some more water through the machine.
As for the shots not coming out as hot, this is a problem thermoblocks face ... learn to live with it! you can trick them a bit by super-heating them by running it on steam for about 5 mins before pulling a shot then running a quick cooling flush, but that becomes really hit-and-miss.