Pretty much all Cro's will read 5-10MHz nicely as old as it may be...

What brand/model is it? i can maybe lookup specs etc...and get a better idea of what controls are on it..

It sounds like your triggering needs to be adjusted, which also may include other associated switches like 'trigger' level' 'trigger polarity', you could switch your input switch (where the probe plugs in) between DC and AC etc etc...