I've been to several mechanics, talked to several friends and posted on several lists without any kind of definitive solution to my problem, but i'll post here incase someone has seen something very similar before.
I have a 91 Toyota MR2 (naturally aspirated) auto transmission with ~80,000mi on it. It's in very good condition for its age, with the interior and exterior looking practically new and all original factory parts. Basically, for a 15+ yr old car it's not too bad.
The problem started a couple months ago. I'd be driving along on the highway and suddenly, the car would start slowing down; imperceptible at first, but it would get hard to maintain the same speed, and more gas would have to be given to the point where I would be slamming the gas down to keep the same speed, and in order not to cause unknown damage to the car i'd slow down and pull off to the side. While slowing down I would put it in neutral and rev the engine, and it would rev without a problem, but put gas and it would feel like it's struggling to move. Applying the brakes made very little difference.
Pulled off to the side of the road, the car is idling ~800rpms and at a full stop - with the transmission in drive. I learned I could press the brakes completely several times, and then the car would creep forward. I could then drive it like normal but the problem would eventually come back, at first about 24 hours at a time and now more like an hour at a time.
I had the fuel filter replaced and it made no difference. I brought it to a Toyota dealership and they found that the brake fluid was a milky-white consistency, and proclaimed this was the cause and please give them $3,000 to replace all my brake parts (hoses, master cylinder, calipers, probably brake booster too and flush the brake fluid). Since I don't have that kind of money lying around, I want to find another way to fix the problem without spending a whole lot. If I can do it in stages that would be best, so if I have to spend $1500 i'd like to do that a little bit here, a little bit there when I get the money.
I understand I should flush the brake fluid first, but what next? (I'd also like to do the fixes myself to save on labor costs, but I am very much a newbie, so if you happen to have the holy grail of automotive repair manuals for beginners lying around i'd love to look at it)
I've gotten these suggestions to fix the problem: test the fuel pump's fuel pressure while driving, replace the cat, rebuild (not replace) the calipers, replace master cylinder/hoses/calipers, check for vacumm leaks, replace vacumm hoses, check/replace vacumm sensor.