Got a '91 4x4 Lariat 6 cyl nicely broken in at 170K miles. Most of the time it shifts smoothly, but as it warms up it occasionally it throws a hard upshift 1->2 or 2->3. 1->2 sometimes is quite violent. Never happens cold, gets worse as it gets warm/hot, but even then it's intermittent. Fluid looks and smells OK. Any optimists out there, or am I headed for a rebuild?