Delta sigma ADC transient simulation in Spectre

[jowong1]

I am running into a problem when I was trying to simulate my DSADC in spectre. The entire system is using ideal components (no transistors, but there are VCVS, R, C) and Verilog models. When I was doing transient sim using "Liberal" setting, for about 60us, it was running fine and giving correct output (noise shaping and all that) but then at about 60us, all of a suddent, there is a "Zero diagonal found in Jacobian at XXXX net and YYYY net". and then the output starts to just oscillate and I cannot figure out why.

One more thing is that my ADC will work ONLY IF I do the transient in "Liberal" setting, if I try to do it in "moderate" or "conservative", then it wont work at all (The output just stays at a constant voltage).

I was wondering if anyone out there can help me with this.......

Thanks