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rough idle/low performance/ cold start & run problems

Fuel System

Vehicle Information: Year: 1986; Engine (Size, Modifications): 930; Total Mileage: 109575;

Vehicle recently had warmup regulator replaced, ran fine for a while - now, starts VERY rough, idles very poorly for first few minutes, and has lagging performance either cold or warm. Checked & replaced vacuum lines to distributor a while back, old ones brittle & cracked, slight change but this condition has started since then. Also, when starting after sitting for a while, clearly hear 'bubbling' sounds never heard before from engine compartment. Checked control line to waste gate and other vacuum/pressure related lines, all OK and boost seems to still be coming on fine - also checked ignition timing, it's OK, and fuel lines & filter - all appear OK. No signs of any vacuum or pressure leaks in the intake air system other than a small amount of oil downstream of the turbo. Also found that the air distribution system from the air pump is not switching to the cold setting and the ignition cold advance is not working, but that shouldn't affect performance when warm.Is this possibly a sensor problem? I know what three of the DME sensors on the engine are for, but there's one in the left timing chain cover right next to the discharge riser from the turbocharger, has two leads to it, that appears corroded/compromised that is quite difficult to see, much less access - what is this sensor for?

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