House has plastic door frames, is this the reason for door problems?

The house is about 40 years old and has always had white plastic doorframes. It doesn't seem to matter how many times we replace the doors, or who replaces them, they always end up becoming 'off' and quite significantly. There are 8 doors in the property and all 8 doors have some sort of problem with shutting or not being straight. Question: Would having plastic doorframes be the problem here or could there be another underlying problem with the walls? How much can I expect to pay if I want to get the plastic doorframes removed and have wooden doorframes fitted? Many thanks.