I have a stumbling problem on my 88 spfi subaru. I have never replaced the fuel pump and it has 150 thousand miles on it however the fuel filter was replaced in april. now when I start it it wont even go past 1000 rpm sometimes unless I go really slow on the gas and not hard on it. before if I just floored it, it would stumble but now it stumbles all the time. what could this be? bad TPS? fuel pump? it did rain a bunch yesterday and I'm wondering if that hurt it or something.

Have you checked the timing lately? My car was doing the same thing and the timing was retarded out of range.