Worth trying the crank sensor - it is the rpm sensor for the ECU and is obviously critical to the fuelling curve, ignition, the lot. If it is wandering or cutting out it can cause the symptoms you describe. There are now a handful of threads on here that renewing this has fixed including me, Doc G and others.
Only about £30 from Alfa and worth doing. But it should be visible as an ECU fault if you want to get the fault codes read first. Look for a Bosch approved garage, they should charge less than £30 allin for reading the fault codes. From memory it's the one of 3 connectors that sit next to each other on a bracket near the bulkhead ; one blue, one brown, one white - one for lambda, one for knock sensor, one for crank, I could be wrong ? You can trace one side of the connector back to the ECU so the other one most lead down towards the crackcases somewhere. I got my garage to do mine but Doc G did his himself - others may be able to advise.