So, here is what I can say I have learnt about volitional + と思っている or と思う .
The first one is used when you have already made some thinking/planning on what you are saying you are going to/ intend to do.
The other one, と思う is used when you make your mind on the spot. This is what my grammar book (genki II)read in short.
7 years ago
That's generally true, but it's a little more complicated than that in practice.
[size=12pt]「週末、泳ごうと思っている。」[/size]I'm thinking of going swimming this weekend.
(like you said, you've already been thinking about it prior to making the statement)
[size=12pt]「週末、泳ごうと思う・・・」[/size]I think I might go swimming this weekend...
(this construction has the impression of a very low chance that you will actually go swimming, and is thus usually continued something like the following examples)
[size=12pt]「週末、泳ごうと思うけど、まだ分からない。」[/size]I think I might go swimming this weekend, but I'm not sure yet.[size=12pt]「週末、泳ごうと思うんだけど、寒いだろうな。」[/size]Well, I think I might go swimming this weekend, but I wonder if it will be cold.
If people really need to show that they just decided something on the spot, they seem to tend to use something like this construction:
[size=12pt]「週末、泳ごうとした。」[/size]I've decided to go swimming this weekend.