Ouch, this isn't going to be much help, I'm afraid, but it sounds like an issue with the SIM card socket. Unfortunately, it's soldered onto the motherboard and would require somewhat sophisticated board level repairs to fix or replace. That sort of repair wouldn't be cheap; from my brief look online it appears motherboards for your phone are running anywhere from about $40 to $150 USD, depending where you get it from, so that might be an option.
Before you go quite that far, it might be worth your time to have a shop take a look at it and see if they can find any issue inside the SIM card slot. They will probably charge you some for the diagnosis, but at least then you'll know what you're working with.
By the way, your phone has a dual SIM card tray; have you by any chance tried the other position for the SIM card?