I had to run a separate line, 20 amp, for my shop. The entire basement originally had one breaker with four outlets. I can use the original line but have to turn off the electric heater in the den to use the shop vac or anything else. For electrical, you can get books at the library or Lowe's that detail what you need to do or get an electrician if you aren't comfortable. If you run the wire outside of the wall like I had to do, you need conduit, gang boxes, and connectors.
Keep in mind that if you install a circuit and it is faulty, your insurance might not cover a claim.
I'm a LLE in TN (limited liability etectrician)( lisc to allow bids/installs up to $25,000.)
In most places a home owner IS allowed to install at his residence BUT IT MUST be inspected to code and also get the permit(s). Some municipalities/states vary to requirements.
1) MOST places require inspection(s)......AND with so many costly claims insurance companies are getting stricter on proper inspections/permits and are starting to deny claims that have yielded not to obey laws/rules.
2) From the info you've given....generally this size box you find in a pre 70'-60's built house and will be approx 800-1,100 sf which usually equals to a mix of more 15 than 20 amp circuits.
3) DON'T attempt a "upgrade/change" from 15-20 amp breaker as most of the time the wire is ONLY rated for 15 amps.
4) Usually a 100 amp box is already to it's max capacity at this day and time ...so a 50 amp seperate/additional box would NOT be advisible
5) IF your already having dimming lights and etc. prior to getting TS there's a possibility you may be losing a leg (weaker current from various things BUT more common ones are loose lugs, wire getting old and weak, corrosion at lug contact points, many , many more) Part of this can be checked by a qualified electrician and in some places the electric company will check from their pole to your meter. I've had them tighten several that over the years had lost that tight bond or their connectors had corroded.
6) I normally DON'T recommend a homeowner (especially as you stated you DON'T know anything about it) to do this type of repairs/install....YES it isn't hard BUT it can be dangerous if not done properly. I AM NOT saying you can't do it.....I'm SAYING I DON'T know your SKILLS!!!!
7) The electrician Should be lisc'd....ALWAYS require permits pulled....IF a electrician ask you to obtain them (permits) then that's a usual sign he isn't lisc'd or certified....you pay the same price as he should, so it doesn't save money as I heard one tell a owner.
8) blades can effect the power draw BUT it's most likely the above.....and yes sappy pine does pull hard also.
9) dry testing (not under a load) the legs sometimes gives false positive (good/working) numbers and once a demand/load is required the numbers drop considerably and usually means a bad connection
Good luck and be safe with your decisions