1991 16 valve hesitant acceleration

[crimp]

Hi everyone, I need your help!
Car has been driving really well for years, then one morning it felt really hesitant and jerky under acceleration on M4. Stopped at service station after 20 miles, put in fuel and jet cleaner and car was fine again. Car was ok for a few days then went crap again. When I drive it somewhere it goes badly, then when it's been left for a while it drives home really well again.
Ignition leads and spark plugs were changed last year - I don't think it's the ignition system and I don't think it's the damp.
Has anyone got any ideas?
   

[tom26]

Just wondering... it may be worth checking for air leaks between the airbox and inlet manifold.
under acceleration the engine can rock allowing air in after the airflow meter, too much air can get in making the car very lean and under powered.

[crimp]

That is what I was thinking, but could not find any. I took out the air filter and put it back, and for some strange reason I removed the main fuel line and put that back. I could not find anything wrong, but when I started the car up and gave it a test drive the car felt realy good with excellent acceleration. I just hope it stays that way.