2004 Mustang GT low and unstable idle, don't know next step, need advice

I recently purchased a 2004 mustang GT convertible. It idles at around 700, but dips constantly to 500 or 600rpm. It idles smoothly if I disconnect the MAS. It does not throw a code. I've replaced plugs, COPs, IAC, air filter, MAF sensor, pcv valve and its hose. I reset the computer a few times. I sprayed carb cleaner over all the hoses (including in passenger fenderwell) and put smoke through the air filter and have found no obvious vacuum leaks. Fuel pressure reading 40PSI at idle. Only thing abnormal on dash is battery light, but battery is new and alternator tests normal, I have an alternator coming this week to replace it just in case, but I think the likelihood of it being the problem is low. Idle issues persist, drives fine otherwise. 97,000 miles.

Any suggestions?
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