Sounds like you need a new mechanic!

The good news is that you might be able to fix this problem yourself. The steering locking up definitely sounds like a problem I had on my 90 240 and 95 850. It is a somewhat common problem on older Volvos. If I am correct, it is a steering shaft (column) u-joint seizing up. Those can seize intermittently as you described.

The steering shaft connects your steering wheel with the rack. The u-joints that connect the shaft to the rack get gunged up and rusty with age and exposure to the elements causing the steering to seize and have some other weird behavior.

There are 2 shaft u-joints. Typically, the lower one (connects to rack) seizes first so focus on that one. Find that u-joint, then using a can of WD-40 spray the u-joint liberally for a couple of minutes while rotating the steering lock to lock. This should fix it, but you might need to apply it a couple of times so be patient. Later you can also follow up with an application of penetrating oil if you wish.

If this is indeed the problem, the worst case scenario is replacement of the offending u-joint.

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Will I buy another Volvo??? We'll see....