We'll assume you've properly cleaned the heads and demagnetized them, right? No rubbing alchol or lemon pledge
.
Make sure you are using new tape. If you changed reels but the second reel was just as old or warn as the first, that's no help.
Also check the rest of the tape path. Clean any metallic contact points such as guide pins and capstan; you could be transferring gook from those to your head.
Check the pinch roller as well. Try cleaning it off with a pencil eraser or special pinch roller cleaner (do not use head cleaner on it.) Is the rubber dry or even crumbly and/or can you see a "groove" where the tape goes? if so, it needs replacement.
Finally, check your heads for rough egdes, grooves, etc. Are the small rectangular heads themselves alingned within the head casing, or are the gaps uneven or is one side of the head assenbly sticking out from the head casing slightly? If any of these are true, you could be shaving the oxide right off the tape and you'd need to replace the heads.
HTH,
G.
