Check battery terminals, and starter motor terminals, are very tight and clean.
If all is good then it may well be the white wire inside the ignition switch, which is easy to solder back on. However, you will need to replace 2 tamperproof screws to get into it. This often fails because too much power is running through it.
At some point, preferably soon, you will need to correct the problem rather than fixing the results of the problem. The best solution is to install the digital keyless remote system made by
@DEcosse . The cheapest alternative can be found in the link in my signature.