What would cause a 1990 legacy to randomly die?

i bought my 1990 legacy wagon for dirt cheap. I had serious motor problems for a while, it
wouldnt idle for a long time until it warmed up, and it had bad misses for a long time.  I used
seafoam through vacuum lines, poured some in tank. That helped some.  I also replaced the
TPS. Everything improved when i replaced the TPS. However if i leave the car sitting idling on
its own, it will randomly die after about 20 minutes.  I also tried leaving a heavy object on the
gas pedal so that it sat around 1500 rpm, and instead of dying when it normally did, the rpm
just dropped from 1500rpm to around 700rpm, and when i removed the object from the gas
pedal the motor instantly died.  Dont understand it.  any help appreciated.  thanks