I have a 97 1/2 ton gmc truck 350ci auto 200k miles runs like new all hwy miles. yesterday on the interstate drowve 50 miles and when I got to my destination it acted like it was in neutral even in reverse I shut it off for a few and started it back up and it was fine for a minute. now it just does the same. somebody told me maybe clogged filter ?

Optional Information:
1997 GMC truck 350

Already Tried:
mechanic over phone/p>