One problem you and your aging Archie may suffer, especially if you've had it apart a few times is 'the curse of the g-string'. No, not that feeling of having your nethers cut in two whilst hunched over your computer! Let's face it you're a retro-computer geek: Fashion means nothing to you, go buy some more comfortable underwear.
No, I refer to that peculiar symptom of erratic boot-ups; weirdness in game-play; keys not working / giving the wrong letter(s); and often seeing a long sting of (usually) 'g's march across the screen when in the command line.
This isn't a power-supply problem, though that usually is the cause of erratic and hard to trace errors, it's your keyboard membrane wearing out.

Usually it's best to give up at this point and buy a replacement membrane for your keyboard but sadly Arc's aren't well served for spares these days and besides why do it properly if you can bodge it yourself? The following method should sort out your issues and needs the minimum of faffing, just a steady hand...

In general there are two main causes of this malady:
The former because you've never had your Arc' apart and I'm afraid stuff just builds up. The latter because you've had your Arc' apart too often and the carbon tracks have rubbed off - see, you can't win!

Build-up can usually be cured by carefully unplugging the connector, give male and female parts a gentle wipe, and put it back together.
If you're tempted to use something like acetone to remove stubborn bits DON'T use it on the membrane tracks and DO wait for the female parts to dry before reassembly. The membrane tracks are made of carbon glued to plastic and Acetone will remove the lot.

Track loss is easy enough to fix, you need to replace the carbon where you can see bits have gone bad. I'd recommend something like Wire Glue applied very carefully with a cocktail stick. You don't need lots, think of the stick as a pencil not a brush.
Allow it to dry (overnight) and your problems should be over.
I'd also suggest running a little of the goop up the tracks away from the connector, not just the obviously bad bits. Keyboard membranes are known for being flexible, less so carbon-in-glue - think of pencils. You can get micro-fractures in the tracks higher-up.

Q: 'What if I go bananas, use too much goo and join some tracks?'
A: 'Wait for it to dry and GENTLY remove the excess with a craft knife.'

Q: 'Do I have to use Wire Glue?'
A: 'No. Lots of carbon, such as lock-smiths use, in nail varnish will do.'
A: 'A PCB track repair pen will also do at a pinch.'

Q: 'Why the link to a commercial web-site?'
A: 'Delivery was super-fast and the cheapest I've seen.'
(credit where it's due)

Here's a tired connector waiting on the treatment

There's no reason why you can't use this method of repair for other membrane tracks but try to keep well away from the keyboard pads. Too much 'lumpiness' will interfere with operation.

Charlie (2009)