My wagon started to behave in a strange way, and I anticipate that more serious problems are on the way.

Somehow idling RPM now become non constant -- I can see with eyes a gentle shake of the tach arrow. Also, it coincides with a very gentle shake of the engine.. this occurs once every minute or so.. Car idles at 1000 and periodically RPM goes down a bit, probably to 900. If you switch on a rear gear and just release the break, car is quite likely to stall. Also, when I resume driving after full stop by gently releasing the break, there is again a feeling that car is about to stall, but this has never happened so far.

Also, this started after my mechanic replaced water pump&all belts, but may be its not related..

 

you SHOULD be idling at about 700rpm. is the throttle returning all the way to closed, and the idle switch on the throttle body activating? if you unplug the idle air controller, your idle should drop to about 500rpm with things completely unloaded (no electrical accessories on, tranmission in park or neutral).

re: mechanic, its very likely related to something he did, its almost always the last thing that was touched. 1987 was a the last bad year for wiring harnesses. is your harness disintegrating? if so, all bets are off.

 

Well, the car was idling like this from the very first time i've installed a tach. I've asked other people if I should adjust it, but reply was that it's ok for such an old car.

So far I didn't have any problems related to harness. There are electrical issues from time to time, but it always boiled down either to fuses or poor electric contact due to oxidation.

Another thin is that after last service done, the car became more difficult to start -- I have to hold the key in the starting position longer till motor kicks in.

 

baloney on the 'for such a old car'. our 87 240 has over 400k miles on it, runs like a new (although my daughter, who took it to Portland OR with her to college, says its become hard to start in cold/wet weather if it sits a week, but as long as she drives every couple days, it starts right up). our 87 is a late 87, and never had any wiring problems (we're the original owners)

 

Check the air intake hose? If the air intake hose came loose it can cause all sorts of wonky problems.

I recently found a crack starting to form in my air intake hose on my '91 240. The engine was starting to hiccup every now and then, this was followed by a scary feeling vibration coming from the engine. After a while I'd have to slam the throttle to get the thing started, and keep the throttle open a little bit at lights and stop signs to keep it from stalling!

If you DO need to replace your intake hose, don't cheap out! I bought a cheap-o $22 part on ebay. Didn't fit right, so I had to struggle to get it on, and one of the crankcase breather fittings broke right off as I was struggling to get it on! After that I spent $43 on one, fit snugly, wasn't a struggle to get on, and didn't break during the install!

 

^ what this guy said. Intake boot, check a/c compressor buttom, if its on it will behave similarly. Things for you to do would be what I did to mine a few weeks ago. New spark plug, cables distributor cap and rotor, new air filter, new flame trap. Don't forget to check you motor mounts and o2 sensor.

 

Could the timing be just a slight bit off causing the hiccups to be cyclical? (OP mentions its every minute or so)

The starting issue could be ignition switch going out or the fuel pump(s) dieing. Has the main pump become noisy?

The air intake hose will not create this issue. I bought my car without one. As far as it can suck in air, there will not be an issue unless under hard acceleration or towing or going up a steep incline etc. Parts are extremely affordable at dealerships given that our cars are old. +1 on not buying junk aftermarket parts.
AC compressor is a non issue. Ditto on the flame trap. However, DO clean/replace the flametrap. Hold the engine oil cap and try to rock the engine. Anything more than a "slight" movement means motor mounts on the way out. Air filter, spark plugs & distributor cap (these 2 should be OEM only) should be changed if there's no history from the previous owner.

Also, dirty/clogged MAF could also be creating issues. Pierce might be able to add what type it is and all that good stuff.

I would clean the throttle body and the idle control valve. Then check for vacuum leaks. And also, check for the fuel pumps. There are DIYs posted/ linked here on how to check if the pumps are working.

And finally, if someone tells you that things rattle or leak or vibrate or RPM fluctuates - that's utter nonsense. I have a 240 (275K miles) & a C70 (305K miles) both of which run just fine.