Hello. I came across your post. I have a craftsman gt300, with kind of the same issue (?).
It'll go in reverse, but not really forward (reg. Speed, under pressure/strain). The oil and filter was changed and it didn't help.
I was wondering if you turning it on its side (on the tire sidewall)...
Hello. I came across this question hoping to see a answer. I'm guessing you didn't get one. I believe that I am also having the same issue. Except that the reverse is fine. Forward is the issue. Here's my e male if could reply, thanks. Lemonlimehomer70@hotmail.com