you are busy with something else
You cannot be busy with ANYTHING else. Not with unreliable airspeed. AVIATE, NAVIGATE, COMMUNICATE.
I have lost count the number of times that I had to ask the PF to fly the aircraft and I will do any thing required heads down. while at low level. Even in the sim with abnormals people just get sucked into the MCDU while PF. Unreliable airspeed means the PF has to set 80% n1 and fly 10 degrees pitch, that is his only job. If the trim is running against you it should be quite obvious right away as it will be harder and harder to fly 10 degrees pitch. If you are trying to do something else at the same time by the time you look at the pitch again it might be too late.