The MA5B is rated for 1,200 watts PEP. 200 watts is certainly well below that threshold. Things to check:
- Arcing, corrosion or moisture inside the match box
- A loose or corroded connection from the matchbox to the driven element
- Loose or corroded connections in the "X hats"
- A waterlogged, corroded or otherwise failed trap
- Loose or corroded connections in the telescoping joints in the elements
- Loose joints between the elements and boom
- Loose or corroded coax connections
- Overheating or voltage breakdown of a balun
I would also recommend substituting a dummy load in place of the antenna to eliminate problems with coax, tuners, SWR meter, amplifier, switches, etc.
If the problem is on a particular band, pay close attention to the applicable traps per point 4 above. The traps have a coil on each end. Water and bug ingresses is a common problem.