microstrip design, with a number of stubs, spurlines

I have a microstrip design, with a number of stubs, spurlines and vias, which occasionally produces a gain of over 100dB at a single frequency. Since there are no devices in the circuit, this is obviously a singularity of some sort in the software. The circuit is all on a single layer, vias only to ground, ports with extensions, and when built it appears to be otherwise accurate. Is this common, what causes it and how can I avoid it?

Normally, there might be some reasons for it. The most common reasons are: 1. Your ports are too close to discontinuities and the accuracy get affected. Normally, such cases may make some strange curves. However, it may not yield 100 dB gain. 2. Your port extensions are closely coupled to the other parts of the structure. It creates a positive feed-back loop. It may cause 100 dB gain if you do not do it properly. 3. There are some very singular cells created in the meshing.