It senses pump operation by the speed with which the thermisters alter resistance, if it happens too quickly it either means that the pump is faulty the thermister/s are faulty (in dhw mode both thermisters are involved in the decision process, as during hot water operation the pcb monitors primary and secondary temperatures) or the boiler is overheating.
So you have many things to eliminate before you buy a pcb.
Might just need desludge/scaling.
There again it could actually be the pcb. For me that is easy to prove as I have one, i would change the thermisters as a matter of course. If you ever see a blue thermister like used in Vokeras on an Alpha change it anyway the present white ones are much better made. The black or grey ones are good. There was another colour that is out of favour but i don't recall what it was, maybe green like aristons, don't quote me. anyhow black grey or white is what you want to see.
Just thinking aloud as it's a grundfos pump head in your shoes I would try that anyway because a grundfos pump is no hardship to put back into stock if you were wrong. So I would try that and try the thermisters, then in my shoes I could also try the pcb as I hold stock.
Actually if dealing with a boiler I don't hold stock for, when I have explored all avenues I do normally order a pcb, but I am normally wrong and now have quite a stock of pcb's which one of these days I will need.
afaik Alpha pcb's are pretty good, though I did change one for a cd32c the other week when the fault said it was fan or flue. I got up a ladder and tookj the flue to bits, changed the fan, and finally had to change the pcb which is what it was. That was a swine, it happens to us all now and again. And of course it's a right parafenalia changing a pcb on a band a boiler. I still have to follow the destructions as I get in a cafudle but I suppose after a while I'll be able to do them off pat and then it'll only take 5 minutes instead of the 1/2 hour it seems to take me right now. I hve also found that some parts changers are not seting them up. When not set up a band a boiler grumbles and can harm itself. I've had to set up a few where someone else who I dare not mention (because local guys do read what I say on here) has changed a part.