I assume -(으)려고 하다 has no point in the end. Probably a comma. (Because those two make a huge difference)
I think it is better to compare between '-려고 하다가' and '-려다가'. Or, '-려고 하다' and '-려다'. In this manner, you can see more clearly that the only difference between those two is 'present tense' and 'continuous tense'.
good point innocence27.
I think there is no huge difference between them.
They both mean "while (being) about to do sth..." "on the point of doing sth...". In the following (subordinated) phrase, it usually explains the reason why that action didn't work out, as innoncence27 has shown you in plenty of examples.