Sorry, meant to say, any ideas
Posting this from mobile phone and have big fingers!

Worth checking the idle speed control valve it might be gummed up and sticking, it is a common fault.
Also check for any air leaks on the induction side as that would definatley cause the symptoms you suggested, have you changed anything since the last time you drove it and it was running ok ?
Good luck
Cheers.....Tony..

If stored outside she could just be damp with recent weather - a good run and warm up could solve. As well as idle control valve I'd check AFM for ease of movement and connections (my connection fell off and gave similar symptoms) and throttle pot though (this normally shows up as high/erratic idle though I believe)

HTH

Mark

For 'AFM' read Air Flow Metres. The big square alloy things with black plastic tops to which the air filter is attached to the front.
Reckon it is just the very damp weather recently and if you clean and dry the connections suggested it will be fixed without more trouble.
Tim

Every time I see your nickname, I think its Steve Heath (writer of the S series bible and builder/ racer of every TVR model that matters)as such I think, why is he asking that!!

Anyway, as I see you are newer to the model I will jump in

I had similar problems recently. Firstly the ECU has an anti-stall which means when the revs hit 600 or lower then the ECU will raise the revs and create the up & down hunting. But why is it that low, I would check the Throttle potentionmeter first as the connections being loose could cause that. If i unplug mine then thats how it behaves.
Next is the tickover air bypass valve, however these are mechanical and generally robust, but they do get gummed up and can be cleaned with carb cleaner.

First things first, do you have a copy of the S series Bible ? so you can identify the parts