Juddering and stalling now when idle.

I'll try and explain this the best I can.

Our MOT was completed in July.

A few weeks ago we took the car to the garage for the air con to be refuelled. I accept this could be a coincidence.

On Monday when refilling the windscreen washer I checked the oil. It had (or appeared to have) dippped below the minimum line and so I got some suitable oil and filled her up. When checking the oil now (cold) it still appears to below the minimum line despite putting a litre of oil in. 

Since the air con was done, my husband had noticed that the car appeared to be slightly more juddery when idle, such as at traffic lights, etc.

I can't honestly say I noticed it. 

On Tuesday, whilst he was driving, he said it felt worse, and I was aware as a passsenger it was a bit jerky, but nothing too serious. When the air con was turned on it improved a bit. HOwever, even when he had the car in neutral and his foot on the brake, the rev counter was going up.

Today I drove the car, locally and whilst running, it is fine. However, I would say it is only OK whilst your foot is on the gas. I was coming down a rather large steep hill and I was aware that it was over revving a bit. At the traffic lights it stalled 3 times, whilst in neutral and handbrake on. 

I got home and opened the bonnet and started the car. The engine block is shaking violently, and yet whilst out of the car and looking at the running of the engine it didn't stall at all.

This car has cost me a small fortune in the last 12 months and I have had my fill of it.

Any ideas?