Hesitation - very strange problem. Insights?

[altmand]

Hello all. I am new to this site, and this is my first post.

I have a 1986 Spider Graduate with 103,000 miles. The car has been in my family for most of its 18 years, and I maintain it religiously, performing 90% of the work myself. The following describes the problem:

When cold, the car hesitates, as if governed, at as low as 3,100 rpm. As the car warms up over several miles, the "governer" raises its limit. There is a sticking point at around 3,900-4,000 rpm, but once the engine is able to rev above 4,000 rpm, it takes only about ten more minutes of driving to have normal access to the entire rev range. However, the first number of trips above 4,000 reveals a sluggish feel as if the mixture is lean.

I have changed the in-tank fuel pump and the fuel filter. The main fuel pump seems to be working fine, and it is not making any noises that it shouldn't. I have checked the mixture, and it is properly set. The car starts every time, and the idle is smooth and steady. Perhaps there is a problem with the variable valve timing solenoid? If so, why would heat affect its performance? Could this be a problem with the fuel pressure regulator?

Any ideas are welcome.

Thank you,

Drew.