What's your charge weight? if it's going a tad fast, perhaps just increase the bean mass a bit? i use 430g per batch and never feel that it would run away like it did for you in roast no. 2.
How do you pre-heat the roaster? i give it plenty of time there. i leave the gas stove on as low heat as possible, the drum spinning and the exhaust closed for about 20 minutes. It then has reached about 200C at which i turn on the heat as high as possible.
during the roast i keep the exhaust always open and actually have the fan running from the turning point on at fairly high to 100% all the way to the end. i used to be much more careful with the fan but that got me a lot of 'body' (bitterness) in the cup. At least for my taste ist was way to bitter... now that i keep the fan extracting all the time i don't see that anymore. However, the more chaff accumulates in the sieve, the less efficient the exhaust fan will be anyway i guess...
It's still to bloody hot here to enjoy roasting though...