This is a long standing problem on Dell notebooks of a certain period, and you'll find plenty of correspondence here if you go back far enough. I'm on my fourth keyboard on my Inspiron 3700, which had exactly the same problem.

People have suggested that the connector  on the keyboard's ribbon cable is the source of the trouble. I'm sceptical about this. Sometimes lifting the keyboard and jiggling the connector seems to help, but I suspect that this is more to do with the flexing of the keyboard that happens in the process. I think the more likely explanation for the problem is heat from the processor damaging the contacts on the keys directly above. One user has suggested removing a raised section on the processor cowl so that it no longer makes contact with the undeside of the keyboard. I tried this, but it didn't help.

Buying new keyboards has become a running expense with my computer. I find each keyboard lasts about a year. I don't think there can be much doubt that the problem is a design fault. If Dell conceded this and said 'We'll knock 10 percent (or some other suitable amount) off your next Dell' I might even contemplate buying another Dell laptop, because there's much that I like about this machine. But I'm so exasperated by this problem that I won't buy another Brand new Dell. I feel that Dell has acted in bad faith.