Ford Mondeo car only running on 2 cylinders instead of the 4

Ah, the ehater and lights draw a lot of current, of which the alternator will need to supply.

Im guessing, but if its when you turn on high power items, heated screen etc, lights, heaters, fans, the alternator is struggling to keep up supplying power, so puts more drain on the engine power.

Have the alternator checked out, it may be the culprit.

Do you have any extras fitted tot he car that would load more power (ie amps, big stereo)?

Turn on the engine then turn off all electrics.. see how it runs. Then turn on all the electrics, lights, fogs, fans, heated screens etc, if the engine starts to struggle is definately a power issue so would definately point to the alternator as mentioned above.