major probs with car

[vegita319]

ok i found out that my ignition timing was far off (near the TDC) and i got that fixed and it runs smoother now. The things is that before i adjusted the timing i was getting around 15 mpg and now im gettin around 10 mpg~! Also my idle is usually 1k (if i go any lower it'll stall after i slow down) and sometimes its at 1k while im at a stop light, it'll suddenly go to ~1800 rpm. This didnt happen before i adjusted my idle. My idle used to be 1200 rpm w/o stalling. I also changed my O2 sensor hoping that it'll help but it didnt help at all. I have a theory that my injectors might be messed up b/c a couple days ago the third cyclind was mifiring and then it worked again the next day. I did a compression test and it was 150-160 on all 4 cyl (is that good?) Can any1 help me with this? btw i searched alrdy and the answers weren't straightforward.