Was the Weimar Republic doomed to fail from the start?

In considering this question we first have to separate the factors that led to Weimar's demise into broadly internal factors, which they had some control over, and external factors, which they did not. For example, considering the Weimar Constitution's system of proportional representation, it is clear that this is an internal, self-imposed weakness that led to a constantly changing host of coalitions coming to power in Weimar Germany, creating a climate of instability. Another example of an internal factor would be the political polarisation in Weimar, with Communist and Fascist parties both holding a sizeable amount of the vote-share and thus meaning that compromise within the Reichstag was very difficult to come by. Then you have to consider the external factors, such as the 1925 Locarno Pact or the fact that Germany was excluded from membership of the League of Nations, these are both things that weakened the Weimar state but were beyond their control, or of course the £6.6 billion in reparations that Germany was expected to pay under the Treaty of Versailles. The internal and external factors combine demonstrate Weimar's weaknesses, however from there you need to make a judgement about whether or not these weaknesses were able to be resolved. For example, Streseman's policy of fulfilment, which involved engaging in diplomatic relations with foreign powers, led to Germany being asked to join the League of Nations in 1926. Furthermore, in December 1924 67% of the vote went to pro-Republic, moderate parties such as the SDP. There is definitely evidence that Weimar's early problems were being resolved gradually by the time of the Wall Street Crash in 1929 that thrust Weimar into the crisis that eventually destroyed it. The magnitude of the economic impact of the 1929 Crash on Weimar was largely down to their dependence on American loans, such as the Dawes Plan or Young Plan, however to say Weimar was 'doomed' from the start is probably an over-simplification, or an overly teleological approach to evaluating the stability of the Weimar Republic overall.