What is the model number of the phone?
Just to prove that it is not a software problem, try starting the phone in safe mode and check if the digitizer (touchscreen) works OK.
If it behaves normally then the problem may be being caused by a downloaded app. The trick is to find out which one.
If it still doesn’t work OK then you have a hardware problem.
If it is a hardware problem it may be caused by a loose digitizer flex cable connection on the motherboard or a faulty digitizer. Was the phone dropped at all?
The phone would need to be opened so that the cable connection could be checked or the digitizer be replaced as it cannot be repaired.
Here’s a link to a video that shows how to replace the digitizer in a Pixi 4. It will most probably be the same for other Pixi models.
Here’s a link to a supplier of the replacement part for a Pixi 4. It is only shown to give an idea of the cost of the part and what it looks like.
There may be other videos on YouTube that show how to replace the digitzer for your model Pixi and also other part suppliers online as well.
Just search for (insert phone model number) digitizer replacement on YouTube to hopefully find repair videos and (insert phone model number) digitizer part in your browser to find suppliers of the part that suit you.
If doing the repair seems too daunting, contact a reputable, professional mobile phone repair service and ask for a quote.