Whilst malfunctioning multirams will result in a noticeable loss of power, your issues sound worse than that, so I would look elsewhere.
Does the car run perfectly normally until warm? How far will it drive before you get problems?
Is the car getting up to normal running temperature ok?
Does the car accelerate OK under full throttle but run lumpy under light throttle?
Also - it's best not to throw new parts at a car with a problem. Find out what is causing it first. A fuel filter, for example, will affect the running whether the engine is hot or cold, so I doubt that is the issue.
I would be checking the coolant temperature sensor first, followed by the Lambda sensors and for any air leaks into the induction and exhaust systems.
Firstly, because the ECU could be fooled into thinking the engine is cold when it's not, hence running an incorrect mixture.
Secondly, when the engine is warmed up, the lambda sensors start to control the mixture, so if anything is fooling them it will affect warm running.
Kevin