What is the light string?
What is the "8.3V" supply?
Do you have to do this with an utterly ancient MOSFET?
What significance is the frequency? What purpose does it serve?
Whatever the frequency, to get useful switching, you need risetime < 1 / (20*frequency). If this is for dimming, you should use an even steeper ratio.
MOSFETs can also switch only in one direction. If this is an AC powered light, you need two MOSFETs. I'm assuming you're switching a DC light here.
Single transistor optoisolators, like 4N25, aren't capable of switching much faster than a microsecond, suggesting a frequency below 50kHz. In a circuit like this (with a weak pull-down and an incredibly massive gate charge), you'll be lucky to succeed at 1kHz. Which I'm guessing is your problem?
Tim