NEED HELP !!!!!!!!

Hi im a newby 2 this forum,but from some of the posts i read here i see there is sum really clever people [unlike myself] lol.The problem i have is a DMTECH [i know BIG mistake] any how its the 26inch dvd combo,i only use this as my pc monitor.i have a nvidia gforce 7300 gs graphics card,i have it connected with the vga{D-Sub it says in book if there is a difference} when i change the screen resoloution to anything above 800x600 i get vertical lines on my screen+whats quite alarming is that the colours are so vivid,not like any colours that you would think a graphics card could make,this even happens when opening a game up,its main title screen opens my tv displays the mode ,ie when opening this game its xga 1024x768,screen gets lines as described above but as soon as the game starts to play it changes back to 800x600 its ok,another game i want to play+have bought lol,runs in 1024x768 and the lines are there all the time.ive tried changing drivers ,refresh rates ect to no avail+scoured the internet for a technical support page + found nothing ,till i stumbled across this forum,although i dont av a clue bout monitors tv's ect i know this isnt supposed to happen,modern technology  lol.Hope some one out there has a few answers that might put this problem to rest. Thanks in advance