You're going to see "flicker" at sweep speeds slower than 1ms/DIV. If there's a front panel holdoff adjustment, it should be set at minimum for the least "flicker".
An obsolete scope? Nah. If it works and it's triggered sweep, it may be old, but not obsolete. I'll take most any Tektronix mid-1970s offering over a new digital model any day. I don't own a digital scope -- used plenty, they're cute, but not worth the price. I have three Teks at my disposal: 213, 465DM44 and a 7904 loaded with plug-ins. I love 'em all!
Dean