But there are two cables, one from the tuner to the switch and the other from the switch to the lnb. Use an adapter to connect the two cables together so they can both be tested together.
If it is an elliptical dish, the further from the center, the signal will drop faster than with a circular dish.
Also, signal level is a very relative term and the same setup connect to different tuners (and even different drivers ) can report big differences for what essentially is the same signal. And as the signal level falls before a certain level, sometimes it just drops to zero because there is not enough to get a sync. SO when adjusting a dish you sometimes see signal go from 90%->85%->70%->0%.
I know you said you tried a different switch, but it may still be the cause. I currently have a faulty switch that took awhile to diagnose. I have two dishes each with a dual lnb. Port 1 from each lnb is connected to my multiple tuner receiver and port 2 from each lnb is connected to a diseqc switch which is connected to pc tuner card. Port 1 of the diseqc switch always worked, but port 2 was hit or miss. Finally I realized the only way port 2 of the diseqc switch worked, was if I was simultaneously using the same lnb on my multi-tuner receiver. So, the dual lnb was not getting enough power through the switch and needed power from the multi-tuner receiver to work. I am not suggesting that is your problem, just that quite often switches do not just fail completely, they exhibit intermittent problems.