cascadeclimbn

Anything else you would recommend "tuning up"? I can figure out how to do most things so if it would keep everything in working order longer then I'd like to know about it. And yes, the tachometer goes up but speed doesnt. I'll have to let it off the gas and press again to even keep going. Its only happened twice out of maybe 20 times on pretty steep, long hills in the cascade mountains but it happens nonetheless. Would that just be a transmission issue or could it be an engine issue as well? Anything I can do to slow the demise of the transmission so I'm not having to replace it in 6 months?

Transmission fluid level and color/viscosity is good. The car still shifts a little jerky when warm but not badly and is a lot smoother than when first driving. Like I said in my OP though, it has over-revved a couple times on steep hills and wasnt getting any extra power when over-revving (might not be describing that well). I'm not sure in what order the codes were retrieved, I didnt know that was important or even a thing so I didnt ask. The guy I bought the car from said he replaced the knock sensor but I don't 100% believe him since he didnt seem like the most honest guy. Is there any way I can check the knock sensor to rule that out if it's working properly? I do not know when the last time the car got a tune up or timing job as there were no records when I bought it and the guy I bought it from said "he" tuned it but again I don't know if his word means much. What would be causing my poor gas mileage (even in fwd and not awd) and could any of these codes be related to that or the 0over-revving issue or the slight burning smell i described? How much does a tune up and full vehicle inspection typically cost. I can generally do a lot of different work on my own cars with a good starting point, help from google, my stepdad's tools, and a haynes manual but thats only b/c im good at following directions and figuring things out on my own...but i dont know a whole lot about cars in general and how to diagnose. The subie beast also has 238k mi on it if that puts anything into perspective.

So I bought a 1996 Subaru Impreza Outback Wagon a few weeks ago. For the most part it runs fine but I've noticed a few problems and the check engine light is on as well. For one it is not getting the gas mileage it should it's getting about 21mpg combined and thats because i have the FWD fuse in...it gets worse mpg when it's in awd like a good subie should be. Second, the engine seems to lack a bit of power and thirdly, which may also tie in with the second, it has over-revved twice on me on steep hills. Fourth, the transmission shifts a little rough especially in the first 10 minutes of driving. And fifth, there is often times (but not always) a slight burning smell coming from the engine bay (but no smoke). I took it to autozone today and had them run the codes on the check engine light and it came back with 3:P0325--faulty knock sensor or circuit bank, P0106-MAP sensor performance (which says likely cause is vacuum leak), and P0301- cylinder 1 misfire detected (leaking or blocked injector). Anyone on this forum good at diagnosing over the computer based on observed issues and check engine codes pulling who can give me an idea as to what could be causing the listed problems and were to start looking at fixing them?