CRJ flight crew on approach to AVL experienced an erratic glide slope indication that the autopilot attempted to follow with increasing pitch oscillations.

Narrative

We were flying a night time; VMC; calm winds approach into AVL. The aircraft was on autopilot and we were established on the localizer and glide slope and proceeding inbound from the marker on a visual approach to Runway 34 at AVL. The glide slope began to act erratically and the autopilot began to follow the glide slope; with progressively larger pitching movements; both up and down. I disconnected the autopilot and manually flew the approach referencing the VASI. The remainder of the approach was uneventful. There is an issue with the glide slope for Runway 34 at AVL. I have experienced this before in different tail numbers and different types of aircraft. Inside of the Broad River NDB; the glide slope periodically begins to give erratic information. In challenging weather conditions; this could lead to compromise in safety; possibly resulting in an accident. The cause of this behavior from the glide slope needs to be identified and addressed. If there is a mechanical issue with the transmitter; even if only a periodic issue; it needs to be addressed to provide predictable; reliable guidance. If there is some sort of ground interference; it needs to be identified. It probably will be difficult to identify why this is happening. However; the combination of mountainous terrain with frequently challenging weather present at AVL could easily elevate this behavior from the glide slope from a nuisance to a causal factor in an accident.