Did anything of note happen when the problem started? Like an overheat or did you just get it back from the garage for another problem?

It sounds like it is temperature related. Will it miss after only 10miles if you let it idle for 20min then get on the highway?

When you say miss, is it more of a stumble or is it a real miss with a severe car shaking misfire? Is it so bad that you are forced to pull over and wait until it recovers before you continue driving?

I have never encountered a problem like this, so I am just feeling my way here. It sounds like you either have a bad seal or gasket that develops a leak after a temp increase, or you have some problem with your wiring.

As far as the wiring goes, pre 1988 240s have a problem with the breakdown of the insulation on the engine wiring harness. If you haven't replaced this yet, this could easily be the cause. Poor electrical signals could yield poor voltages to the injectors or a poor engine ground could lead to poor spark. From Dave Barton a harness is pretty cheap and if you are mechanically inclined it takes about a day to replace it if you work slowly.

In regards to the gasket leak. It could be a severe air leak in the airintake somewhere or it could be a little more major and it could be a head gasket not allowing you to get compression. You should drive around until the problem starts to occure, pop the hood, and listen for an sucking sound on the intake side of the engine to see if you have an vacuum leak. Then do a compression test on each cylander to see if it is the head gasket. If not then you could then check for spark at the plugs, that will see if you have a bad ignition coil or bad engine ground, assuming your new wires and cap are good. Then you might check the voltage going to the injectors to make sure they are getting proper spec. And lastly you might check the timing while you are there. The crankshaft pully has a balancer that does age and can begin to slip. If it slips too much your car will have improper timing and will begin to fire at the wrong time, resulting in a VERY rough run. Checking the timing while the problem is happening should tell you if this is the source of your problems. Although, I have never heard anyone say that this problem could be heat related.

I wouldn't give up on your 240 just yet. I have an '86 as well and at one point I was about to give up because after only doing oil changes for 7 years, it became apperent that a great deal of maintenence was about to be required if I wanted it to last another 170k. The depreiciation of a newer car, not to mention financing interest, that would not require the maintenence of our 18yr old cars would be at least 25times what I spend on parts and tools anually. If you actually spend the money to keep up your car it will run as well as the day it was new, if not better, and it will cost you a fraction of what you would be losing with a new car, with no down payment as well!

Most of the parts you listed are just basic maintenence. Every car needs those. Some of the parts I wonder why your mechanic replaced, like the injectors. Those suckers are expensive and they rarely ever fail or produce poor spray patterns, especially the LH2.2 injectors. I have never heard of anyone that couldn't fix a poor spray with a little techtron. But a mystery can be trying at times. Some other people here might have some better insight than I have. I hope it works out and you report back with the solution to the problem.