I saw an interesting dvar Torah on Noach about "tamim haya bedorosav". The word "bdorosav" is plural meaning that he was "tamim" in all three of the types of situation that he lived in; before, during, and after the flood. That is, he reacted to each circumstance by trying to follow hashem. Before the flood, he reacted to the circumstance of having to live among reshaim and attempting to remain holy. During the flood, as the medrashim state, he was constantly busy maintaining and supporting the animals in the taivah and attempting to keep the remnants of the world alive through the transition. During the aftermath, he became an " ish sadeh", attempting to regenerate the world and restore civilization.
One of the points seems to be that he was unable to maintain the level of "tzidkus" without the full infrastructure that allowed him to be isolated from the rest of the world. In order to succeed at each of the tasks that faced him, he had to throw himself into the role required by that task, and be "tamim" in that role.
To survive in the world before the flood, he had to isolate himself and become a tzadik. This explains the argument as to whether he would have been greater in the time of Avraham or not. Was it a matter of his needing to be isolated in order to survive and reach the level that he did, and he would have been greater in the time of Avraham? Was it a matter of this was the best that he could do and he could not have reached a higher level, so that he would not have been able to reach a higher level in the time of Avraham.
The medrash that he did not sleep for the entire year of the flood, but was constantly busy feeding the animals, cleaning the teivah, etc also hints at this. The appropriate role for that time was one of constant effort. He threw himself into that effort. However, he could not be a "tzadik" in the same sense as he had been before because there was no one to interact with and no opportunity to do anything else.
After the flood, he became an "ish sadeh". Here too in this role he was "tamim". Unfortunately, this was actually a flaw. He was completely a "man of the field". He became so completely a part of this role that when the pressure eased up a little (after the harvest) he became drunk. Here, "tmimus" was actually a flaw. He should have been able to transcend his current role and been a "tzadik" even in that case.
This can also be a different explanation of the phrase "tamim haya bedorosav". He was only able to maintain himself as tamim during the generations (he was 600 at the time of the flood) that he lived among a society that could be considered a surrounding fence. Once the "pressure was off" he could not maintain the same level of behavior. This is similar to the way that many historians (such as Rabbi Berel Wein and Rabbi Dovid Katz) explain what happened after the ghettos were abolished or what happened in the United States. Rabbi Wein has used the phrase "a mile wide and an inch thick". This could be another difference between Avrohom's generation and that of Noach. Avrohom had to fight the society around him and constantly reject the pressure around him to conform. Noach had no pressure whatsoever. He was alone in the world and whatever he did, he would still be the "best". It is like the story of the man who arranged for his daughter to marry the "best boy in the Yeshivah". They married and moved to the town where the father-in-law supported them. After a while, the father-in-law noticed a slackening of the young man's efforts. When he spoke to the boy, he responded that he was learning more and with more intensity than anone in the town. The father-in-law answered that this was not what he had in mind when he said "best". Similaraly, the lack of the pressure that noach had used to keep himself a tzadik contributed to his downfall.