As others have said there is a possibility that the tank can be the cause of the problem. Cleaning and drying the 510 on both the tank and mod each time you change the coil, as you have been doing, helps - but it is advisable to clean the contact inside the tank that touches the base of the coil as well - unscrew the base of the tank and look inside the bottom of it, you will see the contact - give it a good clean taking care not to push this pin out as it can be a complete bastard to put back in.
Quite frankly if you've had this kit since 21 and have been using it constantly then it is probably past its best and due for a replacement. I would buy the same kit again. If you do - test the new tank on your old mod and the old tank on the new mod it will give an immediate answer about if it is your current tank or mod that is the problem. do you have any friends that vape - if you do asking to try to see if their tank works on your mod will quickly show if the mod or tank is the problem.
Stripping and cleaning the mod internals is possible, but unfortunately it isn't guaranteed as if juice has got to the circuit board it can often be terminal so you may have to replace it soon.
If the mod is the problem there are currently Vaporesso Gen Mods for sale at e-cig clouds for 9.99. This is quite an old mod now, but it's a good one and vaporesso tend to be reliable. Probably not as desirable as a new complete kit but it is a decent mod and would be a good stop-gap and back up mod if you get a new kit later.
