Project shows Out of Memory, Dropped Frames, Playback Trouble suddenly.

Hi. I'm suddenly having a lot of problems with a documentary film project in FCP 7, and quite likely all the issues are related. First, I am getting the much feared & dreaded “out of memory” message when in the middle of rendering certain files. Second, am getting the annoying "dropped frames" dialog box when playing sequences. And thirdly, am plagued with things suddenly so sluggish that even when I hit play (space bar) to view a sequence, the command won’t always register, or there’ll be a brief spinning rainbow ball before it even starts playing. I’ve done some googling and read some solutions, but nothing takes so far. The issue is not high resolution photos. Also, I trashed all render and cache files. My internal MBP drive has plenty of free space, but/am using external HD for all media. The sequence settings are 1440x1080 frame size, 23.98 Vid Rate, HDV 1080p24 compression, audio 48KHZ with 32 bit “floating point”. I understand that these are apparently not the best settings, but they are the default whenever I plop my footage onto the timeline, and have spent a couple of years already working on this film, and so that’s what it is. But I haven’t had any problem at all until this week, when I was trying to render a clip that had lots of visual affects on it. It wouldn’t render properly, all kinds of visual disturbances appeared. Finally wouldn’t render at all and gave “out of memory” notice. Then the rest of the project started malfunctioning too. When I ran First Aid on it, it cleared up the whole issue. But when I tried to render this file, again it made a glitchy blotched render, and finally said “out of memory”  and I have the problem all over again. If anybody can take a stab at diagnosis and treatment I would greatly appreciate it. Otherwise I am totally stuck. Thank you!