As a kid I have distinct memories of watching a Spider-Man cartoon on television. Given the house it was in, it could have been as late as 1981, but I'm pretty sure it was really the late 1970s.
I only remember two storylines from the cartoon (and I remember seeing them multiple times, which in hindsight, suggests that this was a budget production with limited material to pull from when broadcasting). One is Spider-Man somehow being transported to an alternate dimension where he fights and escapes from creatures that are essentially sentient plants. The plants - which I'm picturing as somewhat venus-flytrap-like - could move around and communicate and part of the story was Spider-Man prevented them from mounting an invasion of Earth. The plants basically ruled their own dimension and had built a temple where they try to sacrifice Spider-Man, or something like that.
The other one was simpler, it was an out-of-control boulder rolling through a city / suburbia, crushing cars like they're tin cans and doing other damage. In the opening and closing credits and in-between episode segments, it seemed like they used this one repeatedly as a sort of teaser thing (or possibly just to help fill the running time), but I'm pretty sure I saw the actual episode where Spider-Man stops the boulder to prevent more damage or injury.
Those memories are still very clear to me. Less clear is the main bad guy. I'm picturing him in a cloak with a hood; I am not sure if they brought Dr. Doom into that series, or if it was their particular incarnation of Green Goblin or somebody else. I remember the tone of the series being dark and scary, but that might just be from being a little kid. I don't remember any other superheroes ever showing up in the series.
I've tried to figure this one out several times over the years too. It turns out that the Wikipedia entry covering the various Spider-Man television cartoons over the decades is - as far as I can tell - very comprehensive. But going through it, and checking out some possibilities on YouTube and borrowing DVDs from friends, has yielded no matches.