Why is one shot great and the next choked [barista touch]

I recently got a barista touch and so far I have dialed in my beans the best I can. I prefer milk based drinks and always find espresso bitter. I do not have a very refined pallet so for me it is hard to taste the difference in sour and bitter. I am using a local roaster that sells to our local Costco. Filtered water from my fridge and have the filter installed as well. I have my inner burr set to six (tried 4 and couldn't get anything better). My grind is at 12 and my shots are pulling around 28 to 30 seconds. I am using a single basket and do 10 g in and about 25 out. This is the best taste I have found so far (1:2 is way too bitter). However, one time my shot is the best and then next it'll choke. I preheat everything and clean and dry out the Porter filter in between shots. I have two questions.

1. do you think it could be my tamping? as everything else seems to be on par.
2. is it easier to dial in with a double shot? I lost the double basket and I am waiting for a new one