94 2.1 td ... very erratic idle and battery light coming in under 2k revs

Hi guys

My car not doing too well its poorly. Again.

Basics: 94 espace, 2.1td, Elize, 239,000 miles

Started yesterday when I returned from a 190 mile trek (about 130 miles were motorway, the rest country lanes). Car now idling very erratic, jumping from about 200 to 1000 rpm, and I'm not sure if related, battery light is coming on when between 1500 - 2500 rpm but not always at same revs. It depends how harsh I rev. If smooth revving its ok to lower end but if I put foot down it stays on higher revs before going off. I'm more inclined to blame the belt for this as I replaced the alternator about a month ago, although it was from a scrappy I'd possible its duff.

After doing a bit of research I've found this belt only does alternator and coolant, do No idle control out anything to blame so I can't see how they are related but the time its happens is both exactly the same time do I'm thinking it is somehow related

Anyone had similar issues out able to give any pointers of what could be causing?

Also to add, it does seem to have air ingress on the fuel sender in the tank (rough idle and stalls when tank not full)

Thanks in advance

James