Why would a drive shaft disconnect from a gearbox Causing car to slip out of 5th in A Honda Civic EP1 1.4L?

I noticed one day that there was oil underneath where the driveshaft connects to the gearbox. I said it to the mechanic, he said it was ok. Few weeks later the car popped out of 5th eventually it would not stay in. Mechanic said I would need a new box. He installed one and instantly it still popped out of 5th. The mechanic said that where I bought the box had not reconditioned it properly. I drove the car because I had to. A few days later the car didn t pop out of fifth. I taught I may have mended itself. (oil got into all the gears). To my disappointment the car popped out of gear again about 1000 miles later. I brought the car to the garage that I bought the recon box from, I had contacted them the next day after the box was installed and informed them of it slipping out they wanted to see for themselves. The took the car and said that there was Very little oil left in the box as the driveshaft was loose. They refused to replace it. My mechanic then changed the oil in the box. It stopped slipping out for about 100 miles. I check under the car and oil was coming out from the driveshaft. I brought the car to another mechanic. He said it will need a new box. He said the driveshaft won t connect properly. He also said the driveshaft looks fine. Is there any other reason why my drive shaft would come out? Has this ever happened to anyone before? Is my original mechanic to blame? *also steering is heavy in the centre. Pretty sure it s a rusty steering rack or joint. Maybe wrong,

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