let me start by saying that i know all the possible solutions for this problem and will get into those one by one.
But i just wanna start by saying that I'm very disappointed with people using N900.so a teeny tiny problem and N900 is not a good phone anymore huh?AND,to add to the misery,people commenting and posting answers don't know a thing which makes it extremely hard for a new user.people post answers like 'reflash it' or 'take it to nokia care'..For God sakes!
now,the best solution for any problem is IF you know the problem.what i did was,to one by one try out different things that might cause my nokia N900's touch system to go haywire.i found four interesting things.First,the protective cover caused this problem every single time so don't use them.Second,don't install nokia kernel for advanced user if you don't know how to use it.there are other softwares too which caused the touchscreen problems for me.Third,drastic temperature changes cause this problem.for example if you keep the phone right under the AC for a whole night,or keep it under immense heat like direct sunlight.Also,moister from the hands or sweat caused it for me so beware.Fourth and most important,always use N900's original charger to charge the battery.It was surprising for me to see that my nokia's touchscreen developed problems after i charged it with another charger for three nights on a trot.
lets talk about the solutions for all these problems.For first,you just remove the protective cover and the touchscreen will be fine in at most a couple of days.For the second one,be careful that you install only the softwares that you really need,and uninstall the recent ones if you have this problem.For the third problem,give your N900 the optimal temperature and handle it with care and love,and it will be fixed.If there's water or moisture problem,just turn off your N900 and give it two to three days to dry out and then turn it on.For the fourth problem,start charging with the original charger and the phone will be fixed in a couple of charges.By the way,always let the battery drain out as much as possible,let it turn off by finishing off the battery charge if possible,and charge it fully everytime you charge it.
I'm pretty confident that one of these WILL fix the problem as long as its not a hardware problem.The hardware problems include the bad ones like a broken lcd,or a relatively cheaper one like the touch ribbon under the lcd might be displaced or may be the whole touch system has gone bad.The solution for a damaged lcd is to get a new one,the solution for a displaced touchscreen ribbon is to adjust it,and the solution for a 'gone bad' touch system is to change the touch system which also comes pretty cheap.
i absolutely know for sure that this answer WILL help atleast 95 percent of people here.no need to reflash your N900s anymore,there're much easier and simpler solutions available.N900 is undoubtedy the most intersting mobile device that i've used,and i love it to death.Thats why i couldn't just sit back and see people saying bad things about it,and others answering badly and being no help at all.
I'ld like to know what my comment did,how many readers it helped with their problem,and if there're anymore problems,just let me know somehow.i'm always around and would love to make your N900 experience as cheerful as mine.cheers mates.