98 Subaru Legacy

We bought a used 98 legacy and it's engine light turned on. Though nothing of it but lately it has been getting a rougher idle and it recently stalled when in reverse and the gas was hit. Just changed spark plugs what else could give it a vibration-ey feel. Gets about 10km/l thought it would be better so maybe a fuel problem. Every now and then when I start it it struggles, then revs pretty high(no rpm gauge) then goes down to normal. Any suggestions? Sometimes I smell a faint odd smell like burning comming from the vents when the fan is on, but not always and I don't know if it's the car or not.