I was under the impression that Chico topped out at 80% attenuation.
There's really no such thing as a yeast "topping out". The fermentability limit of the wort is set in the brewhouse, and with a low-flocculating strain like Chico, the FG should always be very near that limit.
Just for reference, our standard mash at the brewery is 153.5°F, which gives a target FG of 2.5°P given a 12°P OG. That's 79% ADF, and obviously we could mash several degrees lower to increase that. Normally, I'd suggest that you calibrate your mash thermometer, but given that your results are exactly in line with what I see 4-6 times a week, I think you are actually mashing at about 152°F, and getting the fermentability that would be expected from that mash.
I very much doubt you have a contamination issue. There just isn't enough "surplus" attenuation happening.