re: A/C periodically having very weak air flow

sounds like ice buildup on indoor coil restricting air flow.

during day its hot enough to keep pressures higher so the ice isnt created then or gets melted away as fast as it builds up.

you are low on freon or indoor coil is clogged with dirt and needs cleaning

also, sorry to kill your expectations but just to come out is $100 service charge on top of the $150 plus to diagnose the system issues. you are gonna be lucky if it just costs you $250-$300 just to tell you whats wrong with it and what it costs to fix it. if its just cleaning the coil and freon all in you might be around $500 or a little less if you are lucky. if its 25 yr old and has r-22 freon that shite costs like $50 a pound or more if they even still have it.

my honest opinion is the best bang for your money is bite the bullet now and replace the whole system but thats most likely about a $10k price tag right now

we just had our unit cleaned and checked as routine maint for the summer and it cost us $250, no one is going any where near your unit for just $100, prices that low died many years ago