Hi!
I often put meths as an octane booster, the car runs much better
these cars like good petrol and the higher the octane the better.
This problem sounds fimiliar. What year is the car? And do you know what injection system it has? This engine had 2 different injection systems (well, kind of different) in the Bravos, the early 1.2 had a cable throttle and the later ones had a fly by wire system. We have both systems here and each has its own way of curing an erattic idle. As I don't know what system you have, heres the general cure for both systems:
Cable: The stepper motor is playing up. Try to get a decent second hand one from a scrappie as most have ended up in the scrappie for this reason. Clean thoroughly before fitting as the contats can become caked in corrosion (altho unusual). This sounds like your setup as revs dont seem to be fluctuating.
Non cable: The ecm is relocated to the engine bay on top of the throttle body on this model, originally this was located facing out on the bulkhead. The plugs that go into the ecm, the protecting jacket around them can develop cracks. If this is the case, spray wd-40 or electrical contact spray into the defective plug then seal it up thoroughly and search for further cracks. The plug that gives the trouble is almost always the plug going into the bulkhead
If this dosent cure the problem (although this is the general cause of the problem) the accelerator on the pedal itself or the motorised throttle body is giving bother. Remove these and clean thoroughly with contact spray etc.
Other things that can cause this are the temp sensor and the oxygen sensor, altho these usually give a lumpy idle. On a car that old dirt will have got into almost every crevice you can think of, it may just need a decent clean up.
Hope this helps!