Hi there,
Since yours is an early car the engine systems should be fairly simple, so you should probably go back to basics.
If you've had it for a long time then any faults will have developed recently, it's not like you bought the car with an unknown history and are now trying to debug it.
If the cambelt slipped then it's possible you may have bent a valve, you should do an old-fashioned compression check and hope to get around 180 PSI on all cylinders with the throttle wide open.
The diagnostics should give basic information if anything is wrong, you have changed the single lambda sensor so apart from the water temp sensor there should not be too many inputs to the ECU apart from throttle position and airflow etc. Sometimes throttle bodies need a clean but that is progressive deterioration.
Otherwise I guess you have individual injectors and maybe coils for each cylinder, check these and of course the usual stuff like plugs and leads. Sometimes if you do repairs or maintenance you can disturb an old, failing component and introduce a problem where everything seemed to be fine beforehand.
You don't suggest it is misfiring, so maybe all cyclinders are the same and it's something higher up in the system common to all 4.
Hope it gets sorted soon.
Cheers
John