With a house that old and only 60 amps to begin with, I really think you need to ask an electrician.

try the electrical wiring forum. you might get a bunch of knee-jerk "just drop 25K on a pro, get 200A service and re-wire the entire house," but you might also get some good analysis and advice on how to make the best of what you've got, which is what it sounds like you're trying to do.

I wonder if maybe in the process of changing out bulbs you didn't maybe loosen a wire inside one of your fixtures. If it's been there awhile it could have gotten loosen over the years from a variety of vibrations, perhaps the final straw was changing the bulb.
It could be too that dirt and dust have built up on the fridge and is causing it to work harder. A good cleaning might help.
If it continues after checking the things I've mentioned I'd call a electrician.

you might get a bunch of knee-jerk "just drop 25K on a pro, get 200A service and re-wire the entire house,"

And where I live you wouldn't get insurance on your home until you upgraded it to at least 100 AMP.

Flickering is often normal. But not always. There could be arcing in a box. Heck, I wouldn't be surprised if you had junctions in the wall with no boxes. (The boxes are there to contain sparks and fire.) That's how many ancient homes were electrified. Sparks and centuries old wood make an exciting combination--like kindling and lightning.

When an electric motor starts up - as would be the case with a fridge, sump pump, furnace - it draws a lot more current than it does once it begins operating. So even though you "have reduced the overall load", the new bulbs have not had any effect on the amount of current drawn by the motors on start up.

It's not clear from the op if the lights continue to flicker or if they just do it momentarily when one or more of the motors come on. If it's the latter, it would be the natural reaction of a flourescent light to a momentary drop in current. Why it did not happen before is not clear either. Is it possible that the fixtures the new bulbs were placed in are more visible so you're noticing a flicker for the first time? Are the new places where the bulbs flicker on a different circuit from the older ones that did not flicker? Did the incandescent bulbs you replaced tend to dim a bit when the motor(s) started up?

The lights flicker when the boiler, fridge, sump pump cut in. it is a brief flicker.

I am curious,because the flicker did not happen with the earlier CFL'S but it is happening with the newer ones (additional CFL's were also installed). The 150 watt incandescents never flickered - even with an extra computer running.

The flicker is momentary, a nuisance - but troubling - 'flicker used to be an indicator of inadequate service (from the main)'.

I have reduced the total lighting load by almost 2 kilowatts/day; I have reduced the sump pump load (from 3/4 horsepower to 1/3 horsepower (drainage improvement); the heating system is now 1 boiler instead of 2 furnaces.

I have cut the electrical load as much as I can (without going to LED's) - which are still too expensive!

Is it possible/probable/likely that the Phillips bulbs I used to replace the earlier CFL's are 'less tolerant' of voltage fluctuations?

I have not posted this in other forums, because I think "old houses" is appropriate

It is possible that the newer CFL bulbs are less tolerant of a momentary decrease in current. Easy way to check: Swap one of the CFL bulbs that does flicker with one that does not and see what happens.

As far as total electric usage goes, it would seem as if you should be seeing a decrease, but... Changing the sump pump from 3/4 hp to 1/3 would not automatically result in lower electric consumption. If the efficiency of the 1/3 hp motor is less, you'd wind up with an increase in consumption. Further, the number of times the sump motor goes on is much more important than its output in hp once its running. Same with the boiler. If the pump is running more frequently or is less efficient than the blowers on the old furnaces, you would not be using less total electricity.

Generally, illumination is a relatively minor element in total electric consumption. Anything that heats up - water heater, fridge with automatic defrost, electric range, clothes dryer, etc.- are the big electric users.

I assume that the boiler, at least, is on circuit of its own. Since you're getting the flicker on circuits separate from the boiler's when it goes on, line voltage would be the obvious problem. It does not, however, explain why the problem is showing up at this point.

"I reduced the overall load by more than a megawatt/hour!"

Pretty hard to do with a service that can only deliver 14.4 kW-hours (240 V * 60 A = 14,400 watt-hours = 14.4 kW-hrs).

Induction motors draw many times their running current when starting, and with a small service voltage dips are going to happen.

I would not be surprised if you had either loose or undersized conductors.

At least have someone who knows what they are doing check the major loads.