avionics and autopilot issues

How do you turn on the avionics other than CTL A?With altitude hold on, ILS selected, holding at 3000'msl, when the glideslope becomes active, the aircraft tries to climb to meet the glideslope indicator on the HSI/ADI. The aircraft ended up at 5000 feet and then began descent as the glideslope needle centered. I did have a flight plan active in the FMS. The specific airport was KATL, runway 27R, assigned 3000' with an intercept heading of 300.