riding mower stops moving

I have an old MTD and I am NOT one that knows riding mowers...just trying to fix it without major expense.  And BTW, I am a woman..haha..so please speak dumb to me!

I have an MTD  13AN690G352.  Anyway, when I drive the mower it seems fine until I try to change speeds (by speed control on left hand side) and the mower slows to a stop.  Noticed that the pulleys stop also.  Then I actually got off of the mower and it would not move...as if the brakes were on.  I can get it going again for a few minutes by changing speeds and pressing the clutch, but it stops again.

Help??  Any ideas?  Changed spark plugs, oil and fuel lines...what next....

Thanx