"...am having the same symptoms of the frequency valve cutting off after about 15 seconds."
• That sounds like a symptom of the ECU trying (desperately?) to compensate for a VERY rich mixture.

"Is there something I should check on the ECU or the Lambda control unit?"
• I think of "ECU or the Lambda control unit" as referring to the same thing.
And, yes there are things to check—but I don't know what you have to work with besides a DVM (does it measure "4-cylinder dwell" or "Duty Cycle"?) and a Haynes manual. If your Haynes has "Checks — CIS* Models", starting on page 6-14, I'd like to know what the dwell/dutycycle is doing, with and without the O2 sensor. But that depends on your meter's capabilities.

We also need some background info. For all I know, the car has sat for 10 years and the Fuel Distributor is gummed up. There are ways to check that, but I have no idea if that's a possibility here.
>What led to the seafoam application?
>Was the engine running OK before?
>Has it been out of commission (or running badly) for a long time?
>There must be a back-story here.
>Don't make us keep guessing in the dark.

--
Bruce Young
'93 940-NA (current), 240s (one V8), 140s, 122s, since '63.